Optical transmission network with transmitter photonic integrated circuit (TxPIC) utilizing an optical service channel (OSC)

ABSTRACT

An optical transmitter photonic integrated circuit (TxPIC) comprises a semiconductor monolithic chip with a plurality of optical signal channels where each channel comprises a modulated signal source. The output from the modulated signal sources are coupled to an input of an integrated optical combiner to form a WDM output signal for transmission off the TxPIC chip to an optical transmission link. An optical service channel (OSC) is also integrated on the TxPIC chip to receive a service signal from the optical receiver source which is also coupled the optical transmission link.

REFERENCE TO RELATED APPLICATION

This is continuation of U.S. non-provisional patent application Ser. No.10/267,304, filed Oct. 8, 2002 which claims the benefit of priority ofprovisional applications, Ser. No. 60/367,595, filed Mar. 25, 2002 andentitled, AN OPTICAL SIGNAL RECEIVER PHOTONIC INTEGRATED CIRCUIT(RxPIC), AN ASSOCIATED OPTICAL SIGNAL TRANSMITTER PHOTONIC INTEGRATEDCIRCUIT (TxPIC) AND AN OPTICAL NETWORK TRANSMISSION SYSTEM UTILIZINGTHESE CIRCUITS, now U.S. patent application, Serial No. 367,595; Ser.No. 60/328,568, filed Oct. 9, 2001, entitled, APPARATUS AND METHOD FORFIBER OPTICAL COMMUNICATION, now part of U.S. patent application Ser.No. 10/267,331, filed Oct. 8, 2002; Ser. No. 60/328,207, filed Oct. 9,2001 and entitled, PHOTONIC INTEGRATED CIRCUITS FOR DWDM OPTICALNETWORKS, now U.S. patent application Ser. No. 10/267,331, filed Oct. 8,2002; Ser. No. 60/392,494, filed Jun. 28, 2002 and entitled DIGITALOPTICAL NETWORK ARCHITECTURE, now U.S. patent application Ser. No.10/267,212, filed Oct. 8, 2002; Ser. No. 60/370,345, filed Apr. 5, 2002and entitled WAVELENGTH STABILIZATION IN TRANSMITTER PHOTONIC INTEGRATEDCIRCUITS (TxPICs); Ser. No. 60/378,010, filed May 10, 2002 and entitled,TRANSMITTER PHOTONIC INTEGRATED CIRCUIT (TxPIC) CHIP WITH ENHANCED POWERAND YIELD WITHOUT ON-CHIP AMPLIFICATION, now U.S. patent application,Serial No. 370,345, filed Oct. 8, 2002, all of which applications areowned by the assignee herein and are all incorporated herein by theirreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical telecommunications and moreparticularly to the deployment of photonic integrated circuits (PICs),in particular, optical receiver photonic integrated circuits (RxPICs)and optical transmitter photonic integrated circuits (TxPICs) utilizedin optical transport or transmission networks.

2. Description of the Related Art

The employment of photonic integrated circuits (PICs), also sometimesreferred to as planar lightwave circuits (PLCs), are on the rise inoptical telecommunication systems. These devices provide the integrationof both active and passive optical components on a single substrate andare integrated with other optical components to form a multi-functionaloptical device for use in such systems. The gravitation to PICs isstrong because it leads to utility of providing an entire systemfunction, let alone a component function, in a single chip in a singlepackage. Compared to the deployment of discrete optical components, suchmonolithic PIC chips can significantly reduce the size of opticalcomponents necessary in the optical system, albeit an opticaltransmitter (TxPIC) or optical receiver (RxPIC), for example, as well assignificantly reduce the over cost of the system.

Optical PICs are already known in the art. As related to an opticalreceiver on a chip, the article to M. Zirngibl et al. entitled, “WDMreceiver by Monolithic Integration of an Optical Preamplifier, WaveguideGrating router and Photodiode Array”, ELECTRONIC LETTERS, Vol. 31(7),pp. 581-582, Mar. 30, 1995, discloses a 1 cm by 4 mm PIC chip,fabricated in InP, that includes the integrated components comprising anoptical amplifier (SOA) optically coupled to an AWG DEMUX having aplurality of different signal channel outputs each coupled to arespective photodiode (PD) in an array of on-chip photodiodes. The SOAboosts the multiplexed input channel signals. The AWG DEMUXdemultiplexes the signals into separate channel signals which signalsare respectively detected by the array of PDs. The optical receiver chipmay also be placed on a thermoelectric cooler (TEC) so that the spectralresponse or wavelength grid of the AWG can be fine tuned. A similar PICchip configuration is shown in U.S. Pat. No. 5,913,000 to Doerr et al.but relates to a laser structure without an array of photodiodes, butrather an array of second optical amplifiers in their place, and wherethe PIC chip facets include reflective mirror surfaces to form multiplelaser cavities. Further, an article to C. Cremer et al. entitled,“Grating Spectrograph Integrated with Photodiode Array inInGaAsP/InGaAs/InP”, IEEE Photonics Technology Letters, Vol. 4(1), pp.108110, January 1992, discloses a 4 mm by 7 mm InGaAsP/InP chipcomprising a grating demultiplexer integrated with a photodiode array.The grating demultiplexer comprises a slab waveguide having multipleinput waveguides and output waveguides to and from the slab. The slabhas one end as a reflective mirror and, thus, “mirrors” one half of afull slab waveguide structure. The output waveguides from the slab arerespectively coupled to an array of photodiodes integrated on the InPchip. See also the papers of J. B. Soole et al., Integrated Gratingdemultiplexer and PIN array for High Density Wavelength DivisionMultiplexed Detection at 1.5 mm”, ELECTRONIC LETTERS, Vol. 29, pp.558-560, 1993; M. R. Amersfoort et al., “Low-Loss Phased-Array Based4-Channel Wavelength Demultiplexer Integrated with Photodetectors”, IEEEPhotonics Technology Letters, Vol. 6(1), pp. 62-64, January 1994; and S.Chandrasekhar et al., “Monolithic Eight-Wavelength DemultiplexedReceiver for Dense WDM Applications”, IEEE Photonics Technology Letters,Vol. 7(11), pp. 1342-1344, November 1995.

A combination WDM/PD array is shown in the article of F. Tong et al.entitled, “Characterization of a 16-Channel Optical/Electronic Selectorfor Fast Packet-Switched WDMA Networks”, IEEE Photonics TechnologyLetters, Vol. 6(8), pp. 971-974, August 1994, except that, in the casehere, the InGaAs/GaAs PDs are on a separate chip integrated withelectronic transimpedance amplifiers, selectable switches and outputlimiting amplifier. Light generated from the multiple output waveguidesof a separate AWG DEMUX chip is focused through a lens array to thearray of photodetectors or photodiodes (PDs).

See also the article of B. Glance et al. entitled, “Applications of theIntegrated Waveguide Grating Router”, Journal of Lightwave Technology,Vol. 12(6), pp. 957-962, June 1994, which shows multiple applicationsfor AWG devices with multiple inputs/outputs and their integration withvarious types of active components.

In some of the foregoing disclosures, optical semiconductor amplifiers(SOAs) are employed to boost the incoming channel signals such as froman optical link. Thus, the first on-chip optical component is an activecomponent comprising an SOA to amplify the channel signals. Since thesesignals are of different wavelengths, however, the gain of the SOA isnot equally distributed to all of the channel signals and, as a result,the signals to be amplified do not receive the same gain. This is aproblem because the signals should have substantially equal intensity orpower before they are demultiplexed; otherwise, some of the channelsignals will have significantly degraded BER due to the dynamic range ofthe receiver photodiodes and transimpedance amplifiers.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an optical receiverphotonic integrated circuit (RxPIC) that includes an optical servicechannel (OSC).

SUMMARY OF THE INVENTION

According to this invention, an optical receiver photonic integratedcircuit (RxPIC) comprises a semiconductor monolithic chip having aninput to receive from an optical transmission link a combined channelsignal originating from an optical transmitter source and comprising aplurality of channel signals having different wavelengths forming awavelength grid. An optical decombiner is integrated in the chip andoptically coupled to the input to receive the multiplexed channel signaland provide a decombined individual channel signal on an outputwaveguide of a plurality of such output waveguides provided from theoptical decombiner. A plurality of photodetectors are also integrated inthe chip and each photodetector is optically coupled to one of theoutput waveguides to receive a decombined channel signal and convert thechannel signal to an electrical signal. A controller is coupled toreceive a portion of the converted signals to determine at least oneperformance property of the signals and provide service channel signalas feedback about that property via the semiconductor monolithic chip tothe optical transmitter source. The controller is coupled to anintegrated optical service channel (OSC) on the chip that has a lightsource which is modulated by the service channel signal. The lightsource may be integrated on the chip. The optical service channel (OSC)is coupled as an input to the decombiner for transport off the chip tooptical transmitter source. If there is an optical amplifier at theinput of the chip, the service channel signal will be amplified by thatamplifier. Such an optical amplifier may be an off-chip rare earth dopedamplifier or an on-chip semiconductor optical amplifier.

An optical service channel (OSC) integrated on a TxPIC chip to receive aservice signal from the optical receiver RxPIC chip. Such a TxPIC chipcomprises an optical transmitter photonic integrated circuit (TxPIC)comprising a plurality of optical signal channels each including amodulated signal source with its modulated signal output coupled to aninput of an integrated optical combiner to form a WDM output signal fortransport off the TxPIC chip.

Another feature of this inventions is a method of monitoring andcontrolling a performance property of an optical receiver, such as, butnot limited to, the bit error rate (BER) in an optical communicationnetwork, where the optical receiver in the optical transmission networkmay be a monolithic photonic integrated circuit (RxPIC) chip.

A further feature of this invention is the provision of for a widerwindow margin in an eye diagram of a received electrical signal in atransmission system, such as in an optical communication network.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, the same reference symbols also refer to the same partsillustrated in other drawings.

FIG. 1 is a schematic diagram of an optical receiver photonic integratedcircuit (RxPIC) chip comprising this invention.

FIG. 2 is a schematic diagram of another embodiment of a RxPIC chipcomprising this invention.

FIG. 3 is a detailed schematic diagram of a gain clamped-semiconductoroptical amplifier (GC-SOA) and DEMUX of a RxPIC chip comprising thisinvention where multiple GC-SOAs are provided which also include anintegrated heater for each GC-SOA.

FIG. 4 is a schematic diagram of an RxPIC chip illustrating a furtherembodiment comprising this invention.

FIG. 5 is a more detailed schematic diagram of the RxPIC chip of FIG. 4comprising this invention and illustrating the optical demultiplexer asan arrayed waveguide grating demultiplexer (AWG DEMUX) in the layout ofthe optical components in the RxPIC chip.

FIG. 5A is a detailed schematic view diagram of the GC-SOA/AWG vernierinput which is a feature of this invention.

FIG. 6 is a schematic diagram of an alternative embodiment of thedetailed RxPIC chip layout of FIG. 5 comprising this invention.

FIG. 7 is a schematic diagram of a GC-SOA with greater detail of modeadaptors (MAs) at its input and output.

FIG. 7A is a schematic diagram of a GC-SOA in an RxPIC chip thatincludes an input waveguide to the GC-SOA at an angle relative to thechip input facet to suppress spurious inputs to the GC-SOA other thanthe incoming channel signals.

FIG. 7B is a schematic diagram of a plan view of a GC-SOA in a RxPICchip where the chip includes a curved-shaped GC-SOA with its front endat an angle relative to the chip input facet to suppress spurious inputsto the GC-SOA other than the incoming channel signals.

FIG. 8A is a schematic cross sectional diagram of a specific embodimentof a GC-SOA that may be utilized in the RxPIC of this invention,illustrating an example of the semiconductor layers of the device.

FIG. 8B is a schematic cross sectional diagram of a specific embodimentof a PIN photodiode that may be utilized in the RxPIC of this invention,illustrating an example of the semiconductor layers of the device.

FIG. 8C is a schematic cross sectional diagram of a specific embodimentof an AWG that may be utilized in the RxPIC of this invention,illustrating an example of the semiconductor layers of the device.

FIG. 9 is a plan view of a GC-SOA that may be utilized in this inventionillustrating a segmented electrode contact for applying a bias to thedevice.

FIG. 10 is a plan view of another view of a GC-SOA that may be that maybe utilized in this invention illustrating a tapered electrode contactfor applying a bias to the device.

FIG. 11 is schematic cross-sectional diagram of an embodiment of aGC-SOA that may be utilized in this invention illustrating an example ofthe semiconductor layers of the device with a longitudinal monotonicallyincreasing GC-SOA current channel from the forward end to the rearwardend of the device.

FIG. 12 is a schematic cross sectional diagram of a first type of GC-SOAthat may be utilized in this invention, to wit, a DFB type GC-SOA.

FIG. 13 is a schematic cross sectional diagram of a second type ofGC-SOA that may be utilized in this invention, to wit, a DBR typeGC-SOA.

FIG. 14 is a schematic side view diagram of a third type of GC-SOA thatmay be utilized in this invention, to wit, an external injection typeGC-SOA.

FIG. 15 is a schematic cross sectional diagram of a fourth type ofGC-SOA that may be utilized in this invention, to wit, a VCSEL typeGC-SOA.

FIG. 16 is the first of several figures following this figureschematically illustrating a first embodiment for eliminating ASEgenerated by the GC-SOA from a RxPIC chip.

FIG. 16A is a second embodiment illustrating another approach foreliminating ASE generated by the GC-SOA from the RxPIC chip.

FIG. 17 is a third embodiment for eliminating ASE generated by theGC-SOA from the RxPIC chip.

FIG. 18 is a fourth embodiment for eliminating ASE generated by theGC-SOA from the RxPIC chip.

FIG. 19 is a fifth embodiment for eliminating ASE generated by theGC-SOA from the RxPIC chip.

FIG. 20 is a sixth embodiment for eliminating ASE generated by theGC-SOA from the RxPIC chip.

FIG. 21 is a seventh embodiment for eliminating ASE generated by theGC-SOA from the RxPIC chip.

FIG. 22 is an eighth embodiment for eliminating ASE generated by theGC-SOA from the RxPIC chip.

FIG. 23 is a schematic plan view of an AWG DEMUX having means to providepolarization mode selectivity that may be utilized in this invention.

FIG. 24 is a schematic plan view of an AWG DEMUX having second opticalamplifier means in the form of an array of optical semiconductoramplifiers (SOAs) in the arrayed arms of the AWG DEMUX that may beutilized in this invention.

FIG. 25 is a schematic side view of a type of photodetector foremployment with this invention, to wit, a PIN photodiode (PD) as knownin the art.

FIG. 26 is a schematic side view of another type of a photodetector foremployment with this invention, to wit, an avalanche photodiode (APD) asknown in the art.

FIG. 27 is a schematic side view of further type of a photodetector foremployment with this invention, to wit, an interdigitated contact or MSMphotodetector as known in the art.

FIG. 28 is a schematic side view of a first embodiment of a flip-chipbonded RF submount board with an RxPIC chip comprising this invention.

FIG. 28A is a schematic side view in exploded form of a secondembodiment of a flip-chip bonded RF submount board with an RxPIC chipcomprising this invention.

FIG. 28B is a schematic side view in exploded view of a third embodimentof a flip-chip bonded RF submount board with an RxPIC chip comprisingthis invention.

FIG. 29 is perspective schematic illustration of a wire bonded RFsubmount board with an RxPIC chip comprising this invention.

FIG. 30 is a schematic plan view of architecture for coupling staggeredoutput PD pads formed on the RxPIC which pads are wire bonded tocorresponding input pads on multiple RF submount boards to controlcoupling impedance.

FIG. 31 is a schematic side view of one integrated approach for forminga mode adapter (MA) at the input of a GC-SOA on the RxPIC chip.

FIG. 32 is a schematic plan view of another integrated approach forforming a mode adapter (MA) at the input of a GC-SOA on the RxPIC chip.

FIG. 33 is a schematic view of a first embodiment for eliminating theresidual clamping signal emanating from the GC-SOA.

FIG. 34 is a schematic view of a second embodiment for eliminating theresidual clamping signal emanating from a GC-SOA.

FIG. 35 is a schematic view of a third embodiment for eliminating theresidual clamping signal emanating from a GC-SOA.

FIG. 36 is a schematic view of a fourth embodiment for eliminating theresidual clamping signal emanating from a GC-SOA.

FIG. 37 is a schematic view of a fifth embodiment for eliminating theresidual clamping signal emanating from a GC-SOA.

FIG. 38 is a schematic view of a first embodiment for suppressing thelasing of one of the polarization modes, either the TE mode or the TMmode, in the GC-SOA developed gain clamping laser signal.

FIG. 39 is a schematic view of a second embodiment for suppressing thelasing of one of the polarization modes, either the TE mode or the TMmode, in the GC-SOA developed gain clamping laser signal.

FIG. 40A is a schematic view of a third embodiment for suppressing thelasing of one of the polarization modes, either the TE mode or the TMmode, in the GC-SOA developed gain clamping laser signal.

FIG. 40B is a schematic view of a fourth embodiment for suppressing thelasing of one of the polarization modes, either the TE mode or the TMmode, in the GC-SOA developed gain clamping laser signal.

FIG. 41 is a schematic view of a fifth embodiment for suppressing thelasing of one of the polarization modes, either the TE mode or the TMmode, in the GC-SOA developed gain clamping laser signal.

FIG. 42 is a schematic view of a sixth embodiment for suppressing thelasing of one of the polarization modes, either the TE mode or the TMmode, in the GC-SOA developed gain clamping laser signal.

FIG. 43 is a schematic view of a seventh embodiment for suppressing thelasing of one of the polarization modes, either the TE mode or the TMmode, in the GC-SOA developed-gain clamping laser signal.

FIG. 44 is a schematic plan view of a first embodiment for providingcounter propagating Raman amplification to the optical transmission linkfrom the RxPIC chip comprising this invention.

FIG. 45 is a schematic plan view of a second embodiment for providingcounter propagating Raman amplification to the optical transmission linkfrom the RxPIC chip comprising this invention.

FIG. 46 is a schematic plan view of a first embodiment of an RxPIC chipcomprising this invention that includes a service channel for monitoringoptical characteristics of the channel signals and forwardinginformation back to the optical transmitter.

FIG. 47 is a schematic plan view of a second embodiment of an RxPIC chipcomprising this invention that includes a service channel for monitoringoptical characteristics of the channel signals and forwardinginformation back to the optical transmitter.

FIG. 48 is a schematic diagram of a typical photodiode and atransimpedance amplifier (TIA) configuration as known in the art.

FIG. 49 is a schematic diagram of another typical photodiode and adifferential transimpedance amplifier (TIA) configuration as known inthe art.

FIG. 50 is a schematic diagram of a dual photodiode and a differentialtransimpedance amplifier (TIA) configuration comprising this invention.

FIG. 51 is a partial schematic diagram of the RxPIC chip comprising thisinvention illustrating the utilization of the TIA configuration of FIG.50 comprising this invention.

FIG. 52 is a schematic plan view of a testing scheme of the AWG DEMUXportion of the RxPIC chip employing a thermoelectric cooler (TEC) foroptimizing the peak spectrum of the AWG either through observation ofall the channel signals and/or the differential signal betweenphotodiodes to thermally tune the wavelength grid of the AWG.

FIG. 53 is a schematic plan view of a testing scheme of the RxPIC chipwhile the chip is still in the wafer through the provision a signalinput to the SOA for on-wafer testing the response of the SOA and theAWG DEMUX.

FIG. 54 is a GC-SOA in a RxPIC chip where the gain clamped signal of theGC-SOA is also employed as a counter pump signal for providing gain tothe incoming channel signals propagating on the optical link.

FIG. 55 is a schematic side elevation, similar to FIG. 28, of aflip-chip mounting of a TIA submount to the RxPIC chip comprising thisinvention.

FIG. 56 is an alternate embodiment of a DEMUX that may be employed inthe RxPIC chip of this invention comprising an on-chip echelle gratingor curved grating spectrometer.

FIG. 57 is a bandgap diagram of a tensile-strained MQW region for theGC-SOA in the RxPIC comprising this invention.

FIG. 58 is a bandgap diagram of a tensile-strained active bulk layer forthe GC-SOA in the RxPIC comprising this invention.

FIG. 59 is a schematic plan view of a AWG DEMUX made temperatureinsensitive by having high and low dn/dT arrayed waveguide regions.

FIG. 60 is a schematic plan view of an AWG DEMUX made temperatureinsensitive by employing a region on the waveguide arms that has aheater or current pumped region to change the refractive index of thearms to compensate for wavelength changes. The region can include one ormore regions that include an inserted material having a differentcoefficient of thermal expansion, such as one that decreases orincreases with increasing temperature to provide a more athermal AWGstructure.

FIG. 61 is an alternate embodiment to FIG. 60 wherein there are aplurality heater strips or current pumping regions which can beselectively and/or independently varied to compensate for temperaturedriven index changes of the AWG.

FIG. 62A is a schematic plan view of a first embodiment of a RxPIC chipof this invention employing a photo detection scheme comprising velocitymatched distributed photodetectors (VMDPs).

FIG. 62B is a schematic plan view of a second embodiment of a RxPIC chipof this invention employing a photo detection scheme comprising velocitymatched distributed photodetectors (VMDPs).

FIG. 63 is a schematic plan view of a third embodiment of an RxPIC chipof this invention employing a photo detection scheme comprisingco-planar traveling wave photodetectors.

FIG. 64 is a schematic plan view of an optical transmitter photonicintegrated circuit (TxPIC) and an optical receiver photonic integratedcircuit (RxPIC) optical coupled via a point-to-point opticaltransmission system, with heater control of the DFB laser sources in theTxPIC and heater control of both the TxPIC and the RxPIC AWG devices tooptimize the matching of the wavelength grids of these opticalcomponents.

FIG. 65A is a schematic cross-sectional view of a ridge waveguide,GC-SOA that may be utilized in this invention.

FIG. 65B is a schematic cross-sectional view of a ridge waveguide, AWGthat may be utilized in this invention.

FIG. 65C is a schematic side view of an integrated ridge waveguide,GC-SOA and AWG as shown in FIGS. 65A and 65B.

FIG. 66 is a schematic side view of a GC-SOA and AWG to provide forregrowth to achieve lateral guiding with an index step to optimize forbirefringence at the AWG and maintain single mode guiding in the SOA.

FIG. 67 is a schematic illustration of another embodiment of the RxPICchip comprising this invention where the input channel signals areinitially amplified with a fiber amplifier rather than a GC-SOA.

FIG. 68 is an alternate embodiment of a DEMUX that may be employed inthe RxPIC chip of this invention.

FIG. 69 is an optical-to-electrical-to-optical (OEO) converter utilizingPIC circuits comprising this invention.

FIG. 70 is a schematic view of a forward error correction (FEC) enhancedsystem in an optical transport network with a feedback service channelto correct laser driver current or bias and modulator bias and datasignal waveforms based upon FEC received at the RxPIC.

FIG. 71 is a more detailed schematic view of the RxPIC receiver systemin the optical transport system of FIG. 70.

FIG. 71A is typical desired eye diagram of a channel signal in anoptical transport network.

FIG. 71B is a desired eye diagram of a channel signal in an opticaltransport network of this invention.

FIG. 72 is a flow chart diagram of chirp control in which BER data froman RxPIC shown in FIG. 70 is communicated to a TxPIC or transceiver viaan optical service channel.

FIG. 73 is schematic plan view of an RxPIC chip where the on-chiparrayed waveguide grating (AWG) of the RxPIC is temperature tuned toadjust its passband response to reduce insertion losses deployingmonitoring PIN photodiodes in higher order Brillouin zone outputs of theAWG.

FIGS. 73A and 73B are graphic illustrations of checking the wavelengthgrid passband of the AWG as shown in FIG. 73 relative to a single laseroutput wavelength.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to FIG. 1 which illustrates one feature of thisinvention. FIG. 1 is a diagrammatic view of integrated opticalcomponents comprising the optical receiver photonic integrated circuit(RxPIC) 10 of this invention. RxPIC chip 10 comprises a gain clampedsemiconductor amplifier (GC-SOA) 12 having an input at an input facet(not shown) of chip 10 to receive, such as from an optical transmissionlink, multiplexed optical data signals λ₁ . . . λ_(N) for immediateamplification prior to signal demultiplexing. This is an importantfunction in order to insure that the optical signal to noise ratio orOSNR is maintained at a low noise figure. More importantly, GC-SOA 12 isused instead of a SOA, such as disclosed in the M. Zirngibl et al.paper, supra. A GC-SOA is an amplifier in which feedback is createdthrough an established laser cavity in the amplifier around theamplifying medium so that oscillation is generated inside the amplifiercavity at a predetermined wavelength as defined by a grating formed inthe amplifier cavity. This device is, therefore, a semiconductor laseramplifier having a DFB laser cavity, although the lasing cavity couldalso be a DBR lasing cavity within the scope of this invention. Thereasons why a GC-SOA is better than a SOA in this application is toprovide a gain clamped signal to eliminate loss of gain to higherwavelength channel signals and also the TE/TM gain ratio is fixed due tothe presence of the gain camp signal and, therefore, this ratio does notchange due to power variances in the input channel signals.

As shown in FIG. 1, the output from GC-SOA is provided to an opticaldemultiplexer (DEMUX) 14 where the signal channels are demultiplexed andplaced on DEMUX waveguide outputs as channel signals, λ₁ . . . λ_(N), torespective photodiodes 16 PD(1) . . . PD(N), which produce electricalsignals which are then initially amplified by low noise figure,transimpedance amplifiers (TIAs) 18 as is known in the art. Thepreferred demultiplexer is an arrayed waveguide grating because of itslow insertion loss properties. However, it is within the scope of thisinvention to also include as a demultiplexer, an Echelle grating.

It is within the scope of this invention that, instead of employing anon-chip optical amplifier 12 as shown in FIG. 1, an off-chip fiberamplifier may be employed, such as EDFA 12A illustrated in FIG. 67. Asshown in FIG. 67, this monolithic RxPIC chip 10 would then be comprisedof integrated components comprising, for example, a demultiplexer 14 inthe form of an AWG, for example, and photodetectors 16(1) . . . 16(N).

It is further within the scope of this invention that RxPIC 10 chip 10primarily consists of an AWG and an array of photodetectors which willbe explained in more detail later.

It is also with the scope of this invention that the primary componentscomprising this invention, to wit, a GC-SOA 12, demultiplexer 14(preferably an AWG) and photodetector array 16 be of separate discreteoptical elements. However, it will be understood by those skilled in theart the impact of their integration on a single InP chip to be a highlydesirable, compact, cost effective and easily replaceable component asan optical receiver system.

It is within the scope of this application that photodiodes 16 PD(1) . .. PD(N) may be comprise of a PIN photodiode as shown in FIG. 25, or anavalanche photodiode as shown in FIG. 26, or a metal-semiconductor-metal(MSM) device comprising inter-digitized contacts as shown in FIG. 27.Examples of an avalanche photodiode are disclosed in pending U.S. patentapplication Ser. No. 10/327,362 and entitled, “InP-BASED PHOTONICINTEGRATED CIRCUITS WITH Al-CONTAINING WAVEGUIDE CORES AND InP-BASEDARRAY WAVEGUIDE GRATINGS (AWGs) AND AVALANCHE PHOTODIODES (APDs) ANDOTHER OPTICAL COMPONENTS WITH AN InAlGaAs WAVEGUIDE CORE”, whichapplication is incorporated herein by its reference. Examples of MSMphotodetectors are disclosed in articles of B. D. Soole, et al.,entitled, “Waveguide MSM photodetector on InP”, ELECTRONICS LETTERS,Vol. 24(24), 24 November, 1988; “High-Speed Performance of InAlAs/InGaAsMSM Photodetectors at 1.3 μm and 1.5 μm Wavelengths”, IEEE PhotonicsTechnology Letters, Vol. 2(8), August, 1989; and “InGaAsMetal-Semiconductor-Metal Photodetectors for Long Wavelength OpticalCommunications”, IEEE Journal of Quantum Electronics, Vol. 27(3), pp.737-752, March, 1991, which articles are incorporated herein by theirreference.

FIGS. 62A and 62B provide for alternative type of photodetectors thatmay be employed on RxPIC 10 as compared to the types that have beenpreviously explained relative to FIGS. 25-27 and comprise high speedvelocity-matched distributed photodetectors (VMDPs). VMDPs are opticalwaveguides upon which are fabricated a plurality of photodetectors thatare interconnected with optical and coplanar electrical waveguides andtheir quantum efficiency is dependent upon the number of photodetectorsdeployed in an array along the electrical waveguides. “Velocity-matched”refers to matching the velocity of the RF optics and RF signal along theoptical waveguides. Each of the photodetectors in the array contributeconstructively to an optimum output so that all the photodetectors inthe array must operate in phase with one another relative to any signalchannel in order for an optimum electrical response to be produced. InFIG. 62A, each output channel on a waveguide 39 includes an array ofphotodiodes 270 which produce an electrical signal proportional to theamplitude of the channel signal in a waveguide 39. The electrical signalis collected by a separate microwave transmission line 272 that isvelocity matched to the optical waveguide 39 and the electrical signalsare taken off of chip 10 at pads 274. See, for example, the article ofL. Y. Lin et al., entitled “Velocity Matched Distributed PhotodetectorsWith High-Saturation Power and Large Bandwidth”, IEEE PhotonicsTechnology Letters, Vol. 8(10), pp. 1376-1378, October, 1996, whicharticle is incorporated herein by its reference. In FIG. 62B, TIAs 276and limiting amplifiers 278 are integrated on RxPIC chip 10 employingInP-HBT or InP-HEMT technology.

Reference is now made to FIG. 63 which discloses another type ofphotodetector that may be deployed in this invention comprisingtraveling-wave photodetectors (TWPDs). In FIG. 63, the arrangementcomprises a photodetector 280, such as a PIN photodiode, fabricated atthe end of waveguide 39 from AWG 30. Photodetector 280 is central of aparallel plate, co-planar transmission line comprising signal line 284and ground lines 286 providing for a matched electrical termination atthe output end. As an example, see the article of Kirk S. Giloney etal., entitled “Traveling-Wave Photodetectors”, IEEE Photonics TechnologyLetters, Vol. 4(12), pp. 1363-1365, December, 1992, which article isincorporated herein by its reference.

As shown in FIG. 2, additional amplification of the DEMUX'ed channelsignals can be provided with integrated semiconductor optical amplifiers(SOAs) 20 in the optical waveguides formed between DEMUX 16 andphotodiodes (PDs) 16.

As illustrated in FIG. 3, there may be more than one GC-SOA 12A, 12B and12B at the RxPIC input for purposes of redundancy so that the on-chipperformance of these respective GC-SOAs 12A-12C can be checked relativeto the ITU grid of the DEMUX 14 in order that the best performing GC-SOAcan be selected, e.g., the one with the best gain, saturated power,noise figure, etc. characteristics. The number of SOAs 12 included onchip 10 is preferably in the range of about 2 to 5 such devices. Threeare shown in the illustration here. However, more such devices arepreferred, such a sufficient number to cover or extend slightly beyondthe spectral range for DEMUX 14 where the wavelength variation of thespectral grid in the fabrication of DEMUX 14 may be not be the same forall devices formed in the same wafer or for devices formed from wafer towafer. In this manner, the yield of RxPIC chips 10 obtainable from awafer can be decisively increased. Once the best performing GC-SOA isselected, the input coupling of the channel signals from the fiber linkcan be aligned and fixed to the selected GC-SOA. The selection ofperformance is enhanced also by the employment of heaters 22 placed inclose proximity to each GC-SOA 12A-12C so that the response of theindividual SOAs 20 can be adjusted to better matched to the ITU grid ofoptical DEMUX 14.

Reference is now made to FIG. 4 illustrating the plan view or layout ofa more detailed form of RxPIC chip 10 of this invention which furtherincludes input optical mode adapters (MAs) 24A, 24 b and 24C for therespective GC-SOAs 12A, 12B and 12C as well as respective output opticalmode adapters (MAs) 26A, 26B and 26C. Passive MAs 24A-24C permitmulti-wavelength beam expansion into the GC-SOAs from the single modefiber coupled to one of the selected inputs, while passive MAs 26A-26Cpermit beam reduction to a single mode passive waveguide 37 connectingthe respective GC-SOAs 12 to DEMUX 14. It is preferred that MAs 24 and26 adiabatically increase and decrease the input beam, respectively, inorder that the beam is gradually expanded and then contracted for lowestoptical loss. Also, the use of MAs are critical from the standpoint offorming a composite beam of light that is circular and render it lesscritical in tolerances relative to fiber alignment of the fiber input toRxPIC 10 with regard to input MAs 24. Also, output MAs 26 provide formatching the optical mode from SOAs 12 to DEMUX 14 to insurepolarization insensitivity is preserved with low optical losses andlower optical back reflections such as from downstream opticalcomponents. More will be said about these mechanisms later. Also, shownin FIG. 4 are the contact pads 28 at the output end of chip 10 forreceiving the respective electric signals from PDs 16 for transfer offthe chip to an RF submount board for electrical domain amplification andsubsequent processing.

It would be best to have the alignment of the array of PDs 16 on chip 10to be out of direct alignment of the axial optical path of GC-SOAs 12.Spontaneous emission (ASE generated at the selected GC-SOA 12propagating through chip 10 will provide added noise to photodetectors16. A scheme to spare photoconductors 16 from this noise is illustratedin FIG. 5, next to be discussed.

Reference is next made to FIG. 5 which illustrates in even more detailembodiment of this invention for RxPIC chip 10 comprising thisinvention. Chip 10 is formed in the InGaAsP/InP regime and, for example,may have dimensions of about 1.45 mm by 6.2 mm. Chip 10 includes aplurality of input MAs 24A-24C to expand the input beam at the channelsignal input to a selected GC-SOA 12A-12C, as previously explained, andthe channel signals are reduced to single mode by a respective output MA26A-26C. The input fact to RxPIC chip 10 may include an AR coating, asmay be the case of any of the other embodiments disclosed. The ARcoating aids in coupling multiplexed channel signals into the chip aswell as prevents internal backward reflections from occurring andinterfering with the operation of the chip, particularly the operationof photodiodes 16. The signals are then provided, via a passive on-chipwaveguide 37, to a vernier input of a first slab or free space region 32of AWG DEMUX 30. It is preferred that the length of the MAs be as smallas possible so as not to add to increasing the area real estate requiredfor chip 10. The vernier input shown here comprises three differentinputs to the input slab 32 of AWG 30 so that a best operational matchof GC-SOA 12 to the wavelength grid of AWG 30 can be selected. Thus,through the selection of the best vernier input in the first orderBrillouin zone and the best performing GC-SOA 12, the best wavelengthgrid alignment to AWG 30 can be selected that provides optimizedwavelength matching and lowest coupling loss. This is shown in moredetail in FIG. 5A, which is a bit exaggerated in scale to illustratethis invention. In FIG. 5A, there are five GC-SOAs 12A-12E shownintegrated on chip 10 with their respective output waveguides 37comprising a group of vernier inputs 37V in the central portion of thefirst order Brillouin zone of slab or space region 32. Becausefabricating techniques may not precisely place the amplifier waveguideinput at the exact position desired at the slab 32 input, the placementof a plurality of waveguide inputs along the center of the first orderBrillouin zone forms a vernier permitting the selection through signaltesting of the respective GC-SOAs 12 to determined which first orderinput provides the optimum performance in handing the multiplexedchannel signals, such as in terms of signal separation, low opticalnoise and narrow signal bandwidth. While the best performing GC-SOA 12may be on the wrong waveguide arm to the input of AWG 30 for bestwavelength matching to the grid of the AWG, a lower performance-GC-SOA12 may be chosen in combination with temperature tuning of AWG, via anAWG heater 30A, to optimize the matching of the wavelength grid of AWG30 to the selected GC-SOA.

While other types of optical demultiplexers may be utilized in thisinvention, such as an echelle grating, a multichannel gratingdemultiplexer comprising wavelength-select angled or blazed gratings, areflector stack filter, or multimode interference (MMI) couplers.

A RxPIC chip 10 with a echelle grating type demultiplexer is shown inFIG. 56. Chip 10 comprises an integrated photonic circuit that includesGC-SOA 12, echelle grating 175 and photodetectors 16(1) . . . 16(7).Grating 175 disperses the multiplexed signal wavelengths received fromGC-SOA 12 into separate signals that are reflected back to respectiveoutput waveguides 176 and associated photodetectors 16. See, forexample, the papers of J. B. D. Soole et al., entitled, “WDM DetectionUsing Integrated Grating Demultiplexer and High Density PIN Array”, LEOS1992, Summer Topical Meeting Digest, pp. B7-B8, Jul. 29, 1992 to Aug.12, 1992, Santa Barbara, Calif.; High Speed Monolithic WDM detector for1.5 μm Fibre Band”, ELECTRONIC LETTERS, Vol. 31(15), pp. 1276-1277, Jul.20, 1995; and “Monolithic InP/InGaAsP/InP Grating Spectrometer for the1.48-1.56 mm Wavelength Range”, Applied Physics Letters, Vol. 58(18),pp. 1949-1951, May 6, 1991, all of which are incorporated herein bytheir reference.

A demultiplexer can also be comprised of a series of angled gratingseach of which has a grating period designed to remove from the waveguidea selected wavelength channel from the propagating multiplexed channelsignal. This type of demultiplexer is illustrated in FIG. 68. RxPIC 10comprises a GC-SOA 12 to receive the incoming channel signals, λ₁ . . .λ_(N), which provides signal amplification after which the signalspropagate through mode adaptor 26 and onto a single mode waveguide 182.Waveguide 182 contains a series of angled or blazed gratings 180(1) . .. 180(N), one for each channel signal. Each grating period is designedto have a peak reflection wavelength equal to one of the signalwavelengths, λ₁ . . . λ_(N), so that each of the signal wavelength, λ₁,λ₂, λ₃, etc., is consecutively reflected out of waveguide 182 to acorresponding photodetector 16(1) . . . 16(N) at the same semiconductorlayer level in the chip as waveguide 182. Photodetectors 16 may beformed along one edge of chip 10 to transfer the detected signalsoff-chip. Each of the gratings 180(1) . . . 180(N) may be also providedwith a heater 184 in close proximity to a grating so that the gratings180 may be individually tuned at the factory or in the field to insurethat that their reflection wavelength peaks are at or very close to therespective channel signal wavelength peak for optimum detection at acorresponding output photodetector 16. By the same token, if a channelsignal or signals are slight off their peak wavelengths, the respectivegratings 180 can be selectively temperature tuned to be made closer tothe off-peak wavelength or wavelengths. An advantage of this embodimentas an optical demultiplexer is that the clamping signal and any ASEdeveloped at the GC-SOA 12 is directed forward along waveguide 182 andout of chip 10 providing for high OSNR in signal detection byphotodetectors 16.

A reflector stack filter functioning as a demultiplexer may be of thetype that has plural reflector surfaces that provide for successivereflection of peak wavelengths comprising the channel signal wavelengthsspatially along the filter so that the spatial array of demultiplexedchannels signals may be directed to an array of correspondingphotodetectors (see U.S. Pat. No. 6,111,674 which is incorporated hereinby its reference) or narrow band elliptical mirrors or elliptical Braggreflectors of the type disclosed in the paper of Charles H. Henry etal., entitled “Four-Channel Wavelength Division Multiplexers andBandpass Filters Based on Elliptical Bragg Reflectors”, Journal ofLightwave Technology, Vol. 8(5), pp. 748-755, May, 1990, which paper isincorporated herein by its reference.

A multichannel grating reflector functioning as a demultiplexer isillustrated in the article of P. A. Kirby, entitled, “MultichannelWavelength-Switched Transmitters and Receivers—New Component Conceptsfor Broad-Band Networks and Distributed Switching Systems”, Journal ofLightwave Technology, Vol. 8(2), pp. 202-211, February, 1990, which isincorporated herein by its reference.

An MMI coupler device comprises a multi-mode slab waveguide, which cansupport several modes, with N inputs (in the case here includingdemultiplexing only one input is needed) and M outputs and is based upona self-imaging property wherein an input field profile is reproduced ina single or multiple images at periodic intervals along the propagationdirection of the slab waveguide. See, for example, the articles of LucasB. Soldano et al., entitled, “Optical Multi-Mode Interference devicesBased on Self-Imaging: Principles and Applications”, Journal ofLightwave Technology, Vol. 13(4), pp. 615-627, April, 1995, and of K.Okamoto et al., entitled “Fabrication of Coherent Optical TransversalFilter Consisting of MMI Splitter/Combiner and Thermo-Optic Amplitudeand Phase Controllers”, ELECTRONIC LETTERS, Vol. 35(16), pp. 1331-1332,Aug. 5, 1999, which articles are incorporated herein by their reference.

With respect to all of these different embodiments for an opticaldemultiplexer, an AWG device is preferred because of its better routingand filtering characteristics. Reference is now made to FIG. 5 where anAWG 30 is shown in RxPIC chip 10. As is well known in the art, the slab32 provides for diffraction of a multi-wavelength signal beam into aplurality of outputs to waveguide arms 34, each having a different pathlength. The outputs of waveguide arms 34 are coupled to a second slab orfree space region 36 where the respective channel wavelengths are placerespectively on a plurality of output passive waveguides such as alongthe first order, central Brillouin zone of slab 36. Each of theseoutputs in passive waveguides 39 from WAG 30 is coupled to a respectivePD 16 and the electrical signal output of the PD 16 is placed on arespective output signal pad 28 of chip 10.

GC-SOAs 12A-12C of chip 10 are provided with segmented electrodes orcontacts 32, that may take on a countless number of configurations, butonly a few examples are shown here with respect to FIGS. 5, 6, 9 and 10.In connection with GC-SOA 12A, two segmented electrodes or contacts 32Aand 32B are shown approximately equal distance from the ends of theoptical amplifier. These respective contacts 32A and 32B arerespectively coupled to receive currents, I₁ and I₂. On the other hand,as shown relative to GC-SOA 12B, segmented contacts 32A and 32B arepositioned closer to the output end of the amplifier. Conversely, amultiple number segment contacts 32A-32E may be utilized such as shownin connection with GC-SOA 12C. Segmented contacts 32A-32E arerespectively contacted to receive different currents I₁-I₅. In each ofthese three different cases of GC-SOA contacts, it can be that I₁>I₂ inorder to improve the noise figure (NF) of the amplifier. On the otherhand, if I₂>I₁, then better power saturation, PSAT, can be achieved. Byplacing the contacts closer to the output of the amplifier, such asshown in connection with GC-SOA 12B, improved performance relative tonoise figure (NF) as well as saturation gain might be achieved. In thisconnection, the multiple array of segmented electrodes 32A-32E in GC-SOA12C may be provided with a variety of distributions of applied current,for example a monotonically increasing applied current, I₁-I₅, appliedrespectively to contacts 32A-32E, i.e., I₁<I₂<I₃<I₄<I₅. or adistribution that is relatively larger at both the input and output endsthan in the middle, e.g. I₁>I₂>I₃, I₃<I₄<I₅. The current distribution isoptimized to provide the best noise figure and saturation powerperformance.

RxPIC chip 10 may also have an open trough or layer barrier 38 formed inthe chip body which, for example, extends into the substrate of the chipfor the purpose of blocking or otherwise scattering stray light out ofthe chip, particularly stray light (e.g. ASE) from GC-SOAs 12, so thatsuch stray light does not affect PDs 16, i.e., does not interfere withthe true channel signal sensing to be accomplished by these photodiodesfor demultiplexed channel signals received from AWG DEMUX 30. If barrier38 is an open trough, it is best that its side surface 38A be somewhatrough so that stray light propagating to the trough edge from thedirection of GC-SOA 12 will be scattered out by the rough side surfaceof the trough and out of chip 10. On the other hand, if barrier 38 is tobe filed with a light blocking or absorbing material, a number ofmaterials may be used, which may be semiconductor, dielectric, amorphousor polycrystalline in nature.

Lastly, the input surface of chip 10 is preferably coated at its inputsurface 11 with an antireflecting (AR) material, as is known in the art,in order to increase the input coupling efficiency to the chip and alsoto eliminate spurious reflections from the facet altering the behaviorof GC-SOA 12.

Reference is now made to FIG. 6 where the RxPIC layout provides for lessscattered light interference between optical components but thecompromise is that this necessitates larger chip real estate. However,the area real estate necessary for chip components may be reduced tosome degree by fanning out the bonding pads 28 on chip 10, particularlyfor PDs 16 so that they are positioned along more than one edge of chip10 as shown in FIG. 6, compared to FIG. 5 where they are aligned onlyalong one edge of the chip. Thus, RxPIC chip 10 in FIG. 6 is larger thanthe RxPIC chip 10 shown in FIG. 5 and may be, for example, 4.3 mm by 4.2mm. Also, further shown in FIG. 6 is a SOA 40 and an ASE filter 42formed in each of the output waveguides 39 from AWG 30. SOAs 40 ofselectively controlled, via an applied positive bias, or even negativebias to function as an absorber, to provide for equalization in gainacross the demultiplexed channel signals emanating from AWG 30. Filters42 have a narrow bandwidth within the channel signal spectrum to filterout other wavelengths, particularly higher wavelengths of spontaneousemission or ASE generated in SOAs 40.

It should be noted in this embodiment as well as previous embodimentsthat GC-SOAs 12 generate amplified spontaneous emission or ASE. Anotherreason why AWGs are the preferred choice for demultiplexing channelsignals on-chip is because of the narrow wavelength filter quality ofAWG 30 can filter out the clamping signal associated with the GC-SOAwhich are in the bandwidth of ASE.

It is desirous to keep photodetectors 16 positioned out of direct axialalignment with the GC-SOAs 12 so as to prevent spontaneous emission fromthese devices from being detected by PDs 16. This is accomplished by theplacement of the outputs of GC-SOAs 12 not to be in direct opticalalignment with PDS 16. However, this is not always possible to onehundred percent because the chip size may become too large in thedirection 44 so that it is more apt to break during processing andbecomes more difficult to manage for placement in a hermetic package. Inthe embodiment of FIG. 6, therefore, a trough (not shown) may beutilized in the manner of trough 38 illustrated in FIG. 5.

As indicated in the previous embodiment, multi-segment SOA contacts 32aid to minimize the total current drawn by these devices, particularlyin the upstream portion of the device where the gain should be higher.Moreover, if the saturation power is not required to be too high, thefull bias applied to reach saturation need not be applied at thedownstream end of these devices. The last electrode segment 32C may,alternatively, be employed to measure photocurrent of the GC-SOA 12 tomonitor its gain and correspondingly increase or decrease the gain asnecessary to optimize the operation of the device.

Since the optical power increases from the upstream end towards thedownstream end of a GC-SOA then the optimum use of pump current in aGC-SOA should increase towards the downstream end if it is desired toavoid saturation effects. This can be accomplished in the FIG. 6embodiment by placing less bias on electrode segments 32A compared, forexample, to segments 32B and 32C. Thus, for example, the applied biasmay be controlled such that I₃>I₂>I₁. Alternatively, as shown in FIG. 9,the electrode segments 44 may be monotonically increasing in size towardthe downstream end of GC-SOA 12 so that for equal biases the currentswill vary, such that resulting currents drawn via electrode segments44A-44F, due to their area, would be 44F>44E>44D>44C>44B>44A. Anotheralternative is shown in FIG. 10 where electrode segment 46 is tapered soas to be monotonically increasing in segment contact area from theupstream end to the downstream end of the device. Thus, monotonicallyincreasing current will be supplied along the length of GC-SOA 12.

A further embodiment relating to current control to GC-SOA 12 is shownin FIG. 11 where a cross-section of GC-SOA 12 is shown, in particular atthe region of the GC-SOA at its downstream end. In FIG. 11, GC-SOA 12comprises a buffer layer 62 of n-InP on an n-InP substrate 60 followedby a grating layer 64 of InGaAsP, which is also referred throughout thedescription in this application as a “Q” layer or “Q” multi-quantum wellregion, meaning a “InGaAsP quaternary” layer or region. Also, it shouldbe realized that a semi-insulating substrate, such as InP:Fe or InP:O,may be used instead of semiconductor substrate, e.g., n-InP with anappropriate change in metal contacting as is known in the art. Q layer64 is followed by an InP layer 66 that smoothes out the growth surfacefor the growth of the amplifier active region 65 which is Q layer orquantum well region. This is followed by the growth of confinement layer68 of undoped or p-doped InP. At this point, the initial MOCVD growthprocess is completed, a selective etch is made to buffer layer 62, forexample, and a second growth comprising the growth of current blockinglayer 74 is accomplished which may be, for example, InP:Fe, InP:O orInP:O:Fe, followed by a selective etch though the overgrown blockinglayer 72 to InP confinement layer 68. This isotropic etch step isaccomplished with a tapered mask so that the width of opening 69 formedalong the length of the GC-SOA monotonically increases from the upstreamend to the downstream end of the device, the largest width being asshown in FIG. 11. Then, a third growth is accomplished comprisingcladding layer 70 on p-InP followed by the growth of the contact layer72 p⁺-InGaAs. Thus, the resulting structure is a tapered current channelformed along the optical axis of GC-SOA 12 so that the gain createdthrough contact layer 72 monotonic increases from the upstream end tothe downstream end of the device.

Reference is now made to FIG. 7 which illustrates a plan view of aparticular embodiment for GC-SOA 12 and its optically coupled MAs 24 and26. The similar structural device is shown in the paper of P. Doussiereet al., entitled “1.55 μm Polarization Independent Semiconductor OpticalAmplifier with 25 dB Fiber to Fiber Gain”, IEEE Photonics TechnologyLetters, Vol. 6(2), pp. 170-172, February, 1994, which paper isincorporated herein by its reference. This figure in particularillustrates the waveguide layer of the device where the optical mode ofthe multiplexed signal is initially expanded in input MA 24, amplifiedin GC-SOA 12 (unlike the device in the paper to Doussiere et al.) andthen the optical mode is converted back to single mode via output MA 26for proper mode entry into waveguide 37 and AWG 30. The cavity of GC-SOA12 includes a light feedback mechanism to provide a laser signal at afrequency not within the bandwidth of the channel signals. As will beseen later in FIG. 8A, the feedback mechanism may be a grating. Anothersuch mechanism would be built-in mirrors at the laser signal wavelengthat each end of the GC-SOA cavity.

Reference is now made to FIG. 7A showing another embodiment for GC-SOA12 and MAs 24 and 26. In this embodiment, RxPIC chip 10 includes aninput waveguide formed in the chip for receiving the multiplexed channelsignals, λ₁ . . . λ_(N), from an optical link via optical fiber 46. Tobe noted is that input waveguide 48 is curved relative to input facet52. Fiber 46 is angularly disposed relative to input facet 52 and isaligned to optimize the coupling of the signal into input waveguide 48.It then is mode expanded via MA 24, amplified by GC-SOA 12, modecontracted via MA 26 and propagates on waveguide 37 to AWG 30. In theInP regime, these waveguiding components 48, 24, 12, 26 and 37 mayutilize a light propagating waveguide comprised of InGaAsP or Q, whichis cladded by n-type and p-type InP layers as shown in the paper to P.Doussiere et al. The angularity of input waveguide 48 relative to anormal to the input facet 52 may be several degrees, in the case hereshown as 7°. Input facet 52 also includes an antireflecting (AR) coating54 to reduce stray or scattered light from reflecting back into inputwaveguide 48. Also, the angularly disposed input waveguide 48 at 7°further reduces input facet reflectivity. Curved waveguide 48 to facet52 prevents optical feedback into the cavity of GC-SOA 12. Also,undesired reflections are caused from input facet 52 where optical fiber46 is butt coupled to RxPIC 10 and these reflections can affect theincoming signals, such as, changes in intensity, which is undesirable inDWDM applications.

A further shape for GC-SOA 12 and its associated MAs 24 and 26 may be ofthe type shown in the paper to Hatakeyama et al., entitled, Uniform andHigh-Performance Eight-Channel Bent Waveguide SOA Array for HybridPICs”, IEEE Photonics Technology Letters, Vol. 13(5), pp. 418-420, May,2001, which paper is incorporated herein by its reference. Thewaveguides, comparable to waveguides 48 and 37, as well as coupled MAs,comparable to MAs 24 and 26, are bow-shaped so that the straight SOAsection and the MAs/passive waveguides are smoothly coupled by an 820 μmcurvature. The SOA in this paper, however, is not gain-clamped and isutilized in a different application (switching) and the SOA active layerand the MAs/passive waveguides are one and the same layers.

In FIG. 7B, GC-SOA 12 is also curved with expanding mode adaptor 24provided at input facet 52 to receive the incoming multiplexed channelsignals. The grating for the laser signal of GC-SOA 12 may be providedonly in the downstream section 12G1 or, alternatively, could beprovided, as well, in the upstream curved section 12G2 where thegratings in both sections are normal to direction of light propagationand the pitch of the grating in section 12G2 is greater than that insection 12G1. See U.S. Pat. No. 6,008,675, and FIG. 3, which patent isincorporated herein by its reference.

There are different types of gain clamped-semiconductor opticalamplifiers that may be utilized in connection with this invention. FIG.12 shows a first and preferred type comprising a DFB type GC-SOA 70which basically comprises a plurality of semiconductor layers depositedon an n-InP substrate 72. These layers, in sequence, comprise a n-InPconfinement layer 74, a Q grating layer 76 within which is formed aperiodic grating to cause lasing at a clamp signal frequency, an InPsmoothing layer 78, an active region 80 such as a plurality of quantumwell layers of InGaAsP, which also functions as the waveguide layer ofthe device, a p-InP confinement layer 82, a p⁺-InGaAs contact layer 84,followed by a metal contact 86. Carrier recombination occurs in activeregion 80 to provide for multiplexed signal amplification as well aslasing action at the clamping signal wavelength, λ_(S), for providing aclamping gain function, which wavelength is within the gain bandwidth ofSOA 70 but outside the wavelength grid of wavelengths, λ₁ . . . λ_(N),to be amplified by the SOA. See for example, published patentapplication EP 0639876A1, published Feb. 22, 1995, which is incorporatedherein by its reference.

FIG. 13 illustrates another type of GC-SOA comprising a DBR type GC-SOA90 illustrated in FIG. 13, having a plurality of layers formed on an-InP substrate 92 comprising, in sequence, confinement layer 94,grating layer 96 with distributed feedback grating regions 96A and 96Cand central gain region 96B, smoothing layer 98 of InP, Q region 100comprising an InGaAsP layer or quantum well layers of this quaternary,confining layer 102 of p-InP, contact layer of p⁺-InGaAs, and segmentcontacts 106A, 106B and 106C. See the article of D. Wolfson et al.,entitled, “Detailed Theoretical Investigation of the Input Power DynamicRange for Gain-Clamped Semiconductor Optical Amplifier Gates at 10Gb/s”, IEEE Photonic Technology Letters, Vol. 10(9), pp. 1241-1243,September, 1998, which article is incorporated herein by its reference.With respect to this article, either a DBR GC-SOA with active DBRgrating regions or passive DBR grating regions may be utilized. In thecase of a passive type, contacts 106A and 106C to the DBR gratingregions would not be pumped. However, in the case of an active type,contacts to the DBR grating regions 106A and 106C would be pumped. Thelatter case is preferred because the currents, I₁ and I₃, can be variedto tune the period between these grating regions in order to selectivelytune the laser clamping signal. In this manner, the tuning of theclamping signal can be easily accomplished outside of the amplificationbandwidth of the incoming signals, λ₁ . . . λ_(N), thereby enhancing themanufacturing yield of chips 10 incorporating these types of devices.Current I₂ adjusts the gain of the SOA. Such an arrangement foradjusting the gain can include the adjustment of all three contacts106A, 106B and 106C and also by providing a difference in the currentsI₁ and I₂ with I₁ remaining constant, for example, will also adjust thegain of the amplifier.

It is within the scope of this invention that grating regions 96A and96C in FIG. 11 can be chirped so that through the employment ofmulti-segment contacts 106A, 106B and 106C the tunability range of theclamping signal can be greater over a larger range of tunablewavelengths.

FIG. 14 illustrates a still further type of GC-SOA comprising, in thiscase, a SOA 110. SOA 110 is shown here as a discrete component but isintegrated into RxPIC 10, and is provided at its input with an injectedgain clamping signal along with the multiplexed multi-wavelengthsignals, λ₁ . . . λ_(N), from the optical link. The gain clamping signalmay be optically coupled to the input of chip 10 from an external sourceor, alternatively, may be integrated in a separate region of RxPIC chip10 and optically coupled into the input waveguide, such as, for example,waveguide 48 in FIG. 7A, into SOA 110. SOA 110 comprises n-InP substrate112 upon which is epitaxially deposited lower confinement layer 114 ofn-InP, waveguide core and active region 116 of Q, either an InGaAsPlayer or multiple quantum wells of this quaternary, upper confinementlayer of p-InP 118, and contact layer 119 of p⁺-InGaAs. In thisembodiment, the clamping signal can also be tuned or changed. Also, itis within the scope of this invention to also tune this clampingwavelength to be at a Raman amplification wavelength with counterpropagating of the Raman signal so that it can be coupled out of theRxPIC chip 10 via its input port for coupling into the optical fiberlink and counter propagating therein to amplify the incoming signals, λ₁. . . λ_(N).

Reference is now made to FIG. 15 which is a further embodiment of aGC-SOA 110A that may be integrated into RxPIC chip 10 comprising thisinvention. GC-SOA 110A comprises a vertical cavity surface emittinglaser (VCSEL) having a n-InP substrate 112A upon which is epitaxiallydeposited (following the initial deposit of a n-InP buffer layer) astack of semiconductor DBR layers comprising bottom mirror 127,confinement layer 116A of n-InP, Q waveguide layer 118A, spacer layer120A of InP, active region 122 comprising a Q layer or a Q-QW region,confinement layer 124 of p-InP, a stack of semiconductor DBR layerscomprising top mirror 128 and a contact layer 123 of P⁺⁺-InGaAs. Bottomand top DBR mirrors 127 and 128 may be comprised of 20 to 50 layers ofInAlGaAs layers of alternating different mole fractions, or alternatinglayers of InAlAs/InGaAs or InAlGaAs/InP or InGaAsP/InP. This is followedby the necessary metallization comprising p-side metal contact 125 andmetal contact 129. Operation through current and bias of GC-SOA 110Aprovides lasing action producing gain clamping signal 117A betweenmirrors 127 and 128 providing gain at active region 122 foramplification of the channel signals, λ₁ . . . λ_(N), propagating inwaveguide layer 118A. An advantage of deploying VCSEL GC-SOA 110A isthat the portion of the gain of clamping signal 117A not utilized canexit the chip via the top or bottom of amplifier 110A since DBR mirrors127 and 128 are not 100% reflective. On the other hand, amplifier 110Ais more difficult to fabricate than many of the other embodimentsdisclose herein. There are several other ways of eliminating theunutilized gain of the clamping signal which will be described latter.

There is also a Mach-Zehnder (MZ) type of SOA that may be utilized inthis invention. In this case, the clamping signal can be coupled intothe Mz-SOA.

It should be noted that in all of the forgoing embodiments of thisinvention, the gain clamping signal can be either on the long wavelength(red) or short wavelength (blue) side of the bandwidth or wavelengthspectrum of the multiplexed channel signals.

Reference is now made to FIGS. 8A, 8B and 8C which respectively discloserepresentative InP-based embodiments, in cross-section, comprising a DFBGC-SOA 120, a photodetector, shown here as a PIN photodiode 16, and agrating arm 34 of AWG 30. In FIG. 8A, DFB GC-SOA 120 comprises a firstepitaxial growth process on n-InP substrate 112 upon which isepitaxially deposited in sequence a n-InP buffer layer 124, a lowerconfinement layer 126 of n-InP, Q grating/waveguide layer 128 (bandgapof about 1.3), an InP spacer layer 130 (bandgap of about 1.4),active/waveguide region of Q-QW layers 132 (bandgap of about 1.5), andupper confinement layer 134 of p-InP. Next, a selective etchback isperformed with masking of the yet to be defined current confinementregion 129, followed by a second epitaxial growth process comprisingblocking layers which comprise a first layer 131 of n-InP followed bysemi-insulating (SI) layer 133 of InP:Fe, InP:O or InP:O:Fe, and a thirdlayer 141 of p-InP. Masking over the current confinement region 129 ofthe formed device is removed and a third epitaxial growth process iscommenced comprising cladding layer 135 of p-InP, followed by contactlayer 136 of p⁺-InP, which may be optional here but is desired elsewherein the integrated chip, and thence contact layer 138 of p⁺-InGaAs.Device 120 is completed with p and n electrodes 138 and 139. The channelsignals propagate along active layer 132 with evanescent overlap withwaveguide layer 128 and these signals absorb gain and are amplified.Layer 128 also includes a DFB grating 128A for generation of the gainclamping signal.

It should be realized that the fabrication of GC-SOA 120 is done inconjunction with other optical components included in integrated form onRxPIC 10. Thus, the epitaxial fabrication sequence in the descriptionhere may be modified or include other steps so that the layers andstructures of such components can be also be added or otherwiserealized. This same statement also applies relative to the structuresshown in FIGS. 8B and 8C.

In conjunction with the GC-SOA 120 of FIG. 8A, reference is now made toFIGS. 31 and 32 which illustrate input mode adaptors or converters 24that may be employed for GC-SOA 12 or 120. In this connection, withreference to FIG. 31, the mode adaptor taper 24 is in the vertical planeof PIC chip 10 whereas, in FIG. 32, the mode adapter 24 is tapered inthe horizontal plane of PIC chip 10, i.e., in the plane of its as-grownlayers. In FIG. 31, generally the same layers are shown as in the caseof previously described FIG. 8A. However, with a selective maskingtechnique, such as selective area growth (SAG), taper 24 may be formedin Q active region 132 during its growth to provide for an adiabaticexpansion of the input light comprising channel signals, λ₁ . . . λ_(N).In this connection, see U.S. Pat. No. 6,141,477, which patent isincorporated herein by its reference. Patent '477 illustrates such ataper at the output end of an active region rather then the input end ofan active region. Such an output taper can be also employed at theoutput end of GC-SOA 12 of FIG. 31.

In FIG. 32, the input tapered mode adapter 24 is shown in the horizontalplane. In fact, tapering can be provided in both the vertical andlateral directions to provide an input or output mode adapter 24 or 26that adiabatically transform the input or output light as taught in U.S.Pat. No. 6,174,748 relative to output light, which patent isincorporated herein by its reference. In patent '748, the purpose is toproduce an output beam of substantially circular mode profile, whichneed not be the case here as long as the multiplexed channel signals areadiabatically converted to single mode.

Reference is now made to FIG. 8B which illustrates a cross-section of aphotodetector that may be utilized in RxPIC chip 10. Photodetector 140is a PIN photodiode comprising multiple epitaxially deposited layers,some of which are not necessary to its operation but; present because oftheir necessity for other optical component(s) on the same monolithicchip 10. The structure comprises, as is the case of GC-SOA 120, a firstepitaxial growth process of n-InP substrate 122 upon which isepitaxially deposited in sequence a n-InP buffer layer 124, a lowerconfinement layer 126 of n-InP, Q layer 128 (bandgap of about 1.3 withno grating 128A provided in this portion of the chip), an InP spacerlayer 130 (bandgap of about 1.4), active/waveguide region of Q-QW layers132 (bandgap of about 1.5), and upper confinement layer 134 of p-InP.Next, a selective etchback is performed with masking of the yet to bedefined current confinement region 129, followed by a second epitaxialgrowth process to form current blocking layers comprising a first layer131 of n-InP followed by SI layer 133 of InP:Fe, InP:O or InP:O:Fe and athird layer of p-InP. Masking over the current confinement region 129 ofthe formed device is then removed and a third epitaxial growth processis commenced comprising cladding layer 135 of p-InP. This is followed bythe deposition of contact layer 136 of p⁺-InP and dielectric passivationlayer 137. Contact layer 136 is defined by dielectric layer 137 whichmay be SiO₂ or other such dielectric. The device is completed with ap-side metal contact 138 and a n-side metal contact 139. It should benoted that GC-SOA 120 is substantially the same as PIN photodiode 140except that waveguide layer 128 contains no grating at photodiode 140. Qlayer 128 at photodiode 140 functions as a carrier depletion device bybeing reversed biased and generating a current signal proportional tothe light entering the photodiode. Also, as will be realized from theprevious description of RxPIC chip 10, there are N such photodiodes 140(or 16) formed on the chip.

Reference is now made to FIG. 8C which illustrates the epitaxiallydeposited layers for AWG 130 such as AWG arrayed arms 34 as well asrepresentative of cross-sections of input and output waveguides 37 and39. FIG. 8C represents an exemplary cross-section of any of thesewaveguide structures comprising AWG 130. The slab or space regions 32and 36 of AWG 30 would have the same composite structure except thewaveguiding region would have a larger extent in the lateral direction.As in the case of the optical components 120 and 140 in FIGS. 8A and 8B,a first epitaxial growth process on n-InP substrate 122 comprisesepitaxial deposition, in sequence, a n-InP buffer layer 124, a lowerconfinement layer 126 of n-InP, Q waveguide AWG layer 128 (bandgap ofabout 1.3 with no grating in this portion of the chip), an InP spacerlayer 130 (bandgap of about 1.4), active region of Q-QW layers 132(bandgap of about 1.5 but having no direct function in AWG 30), andupper confinement layer 134 of p-InP. Next, a selective etchback isperformed with masking to define waveguide structure 127, followed by asecond epitaxial growth process comprising cladding layers (inpreviously described structures of FIGS. 8A and 8B functioning asblocking layers) comprising a first layer 131 of n-InP followed by SIlayer 133 and a third layer 135 of p-InP. It is within the scope of thisinvention that waveguide 127 be not covered, i.e., it can be anair-exposed waveguide or, alternatively, other layers may be utilizedfor burying waveguide 127, such as, SiO₂, glass (silica), BCB, ZnS orZnSe as examples.

The preceding described embodiments of FIGS. 8A (GC-SOA), 8B (PD) and 8C(AWG) are examples of buried types of devices. On the other hand, thesedevices can be deep ridge waveguide devices as illustrated, for example,in FIGS. 65A, 65B and 65C. GC-SOA 330 in FIG. 65A, along with AWG 350 inFIG. 65B, comprise a n-InP substrate 332 upon which are epitaxiallydeposited n-InP confinement layer 344, Q-grating/waveguide layer 336(with grating layer 336A and smoothing layer 336B), and undoped InPlayer 338. This is followed by the growth of active region 340 in GC-SOA330 employing selective area growth (SAG) so as to taper this layer at331 as shown in FIG. 65C. This is followed next with the growth of p-InPconfinement layer 342 over both GC-SOA 330 and AWG 350. Then an etchbackis performed to form the deep ridge waveguide structures as shown inFIGS. 65A and 65B. Note that the ridge is formed back through thewaveguide core 336 to provide for birefringence at the AWG and ease ofmanufacturability, i.e., providing for less epitaxial growth steps.These structures may be exposed to air or covered with a high refractiveindex material such as BCB, ZnS or ZnSe.

To be noted in FIG. 65C, the waveguide layer 336 is continuous. However,through several epitaxial growth steps, the regrowths provide forlateral guiding with an index step optimized for birefringence at theAWG and single mode guiding at the GC-SOA or the photodetector. In thisconnection, reference is now made to FIG. 66. FIG. 66 shows alongitudinal cross-section of RxPIC 10 comprising input mode converter400, GC-SOA 360, AWG 380 and PIN photodetectors 390(N). A typicalprocess for fabrication of this structure is a first growth processcomprising the deposition of a n⁺-InP buffer layer 364 followed by thedeposition of a Q-grating layer 366, followed by an n-InP cap or stopetch layer (not shown due to subsequent removal). Next, a selective dryetch employing a photoresist mask is made in Q layer 366 to form DFBgrating 367 for GC-SOA 360. Next, a second growth process is initiatedcomprising an undoped-InP planarization layer 368 to planarize overgrating 367. This is followed by a third growth process comprising thedeposition of Q-waveguide layer 370 and thence an n-InP cap or stop etchlayer (not shown due to subsequent removal). Then a photoresist mask isapplied to the area of AWG 380 and the AWG is defined via selectiveetching. Then, waveguide layer 370 over the regions comprising GC-SOA360 and PIN photodetectors 390 are etched away. This is followed by afourth epitaxial growth process for overgrowing the grating 367 as wellas forming active region 374 for both GC-SOA 360 and PIN photodetector390. First, an undoped InP layer 372 is deposited followed by a Q-activelayer or MQW active region 374, followed by p-InP layer 376 and contactlayer 378 of p⁺-InGaAs. These layers 372, 374, 376 and 378 are thenetched over mode converter 400 and AWG 380. Subsequent processingprovides for a buried waveguide structure such as shown in FIGS. 8A, 8Band 8C or a ridge waveguide structure such as shown in FIGS. 65A and65B. The etched regions 382 over mode converter 400 and AWG 380 may beoverlaid, for example, with InP:Fe, InP:O, BCB, ZnS or ZnSe. Arrow lines385 in FIG. 66 shows the path of the channel signal mode as itpropagates through the one illustrated signal channel of RxPIC 10.

FIGS. 16-22, 37 and 54 relate to various ways of either eliminating theamplified spontaneous emission (ASE) or residual laser gain clampingsignal, or both, from RxPIC chip 10. The residual gain clamping signaland ASE generated by the amplifying function of GC-SOA 12 or 120 isundesirable on chip 10 as it will interfere with the accurate detectionfunctioning of photodiodes 16 or 120. When current is injected intoGC-SOA 12 or 120, ASE is emitted which is optical noise that interfereswith the detection response of photodetectors 16 as well as providingreflected light back into GC-SOA 12. Also, the DFB grating generatedlaser light to maintain the gain of GC-SOA 12 is not totally utilizedand, therefore, propagates out of the amplifier to AWG 30. These figuresillustrate approaches to eliminate this noise from RxPIC chip 10.

In FIG. 16, reliance on AWG 30 per se to filter out this noise isselected. In the case here, the AWG 30 must be designed to function assuch a filter so that the cone filter function is limited strictly towavelengths within the wavelength grid of the channel signals.Wavelengths outside this spectrum, such as ASE at higher frequencies ora laser clamping signal at a higher or lower wavelength not within thisspectrum, is rejected by the narrow band filtering of AWG 30. In thiscase, AWG 30 must be designed to have a large free spectral range (FSR)to filter out the GC-SOA clamping wavelength signal.

As illustrated in FIG. 16A, the input waveguide 37 from GC-SOA 12 can beprovided with a high angular bend at 37A along which the signalwavelengths can be guided but not the higher wavelengths of ASE or of again clamping signal if of sufficiently higher wavelength than that ofthe channel signal spectrum. Otherwise, where the laser gain clampingsignal is a shorter wavelength than the channel signal spectrum, othermeans may be necessary to remove this signal from chip 10.

The free spectral range (FSR) of AWG 30 can, thus, be designed to filterthe gain clamping signal propagating from GC-SOA 12 to AWG 30. If thewavelengths of the modes are very different, then the FSR of AWG 30 willhave to also be large as well. This can therefore become a designconstraint for the AWG. Thus other measures will have to be taken to ridthe RxPIC chip 10 of this residual clamping signal, which is to bedescribed in several subsequent embodiments.

FIG. 17 illustrates another way of filtering out ASE and the laser gainclamping signal. In this illustration, a bank of on-chip SOAs 31(1) . .. 31(N) are integrated in each of the output waveguides 39 of AWG 30 toprovide immediate gain to the demultiplexed channel signals which haveexperienced some insertion loss. These SOAs 31 are followed by in-linefilter devices 33(1) . . . 33(N) which may be on-chip angled or blazedgratings with broadband reflective gratings within the bandwidth of theclamping signal and ASE to, not only eject the ASE and clamping signallight from GC-SOA 12 from chip 10, but also eject the ASE generated bySOAs 31. Thus, these unwanted wavelengths which are noise are eliminatedfrom proceeding on with the demultiplexed channel signals tophotodetectors 16(1) . . . 16(N).

FIG. 18 is a further embodiment for rejection of ASE and the residualgain clamping signal employing architecture similar to FIG. 17 exceptthat the input amplification to the multiplied channel signals is aRaman or a rear earth fiber amplifier 35, such as an EDFA, as opposed tothe employment of GC-SOA 12 or 120. Here, instead of on-chip initialamplification via GC-SOA 12 of the multiplexed channel signals, anoff-chip booster fiber amplifier 35 is utilized. (This is similar toFIG. 67, previously described). In the case here, SOAs 31(1) . . .31(N), shown in FIG. 18, are optional. In-line filters 33(1) . . . 33(N)each have a filter band that passes the demultiplexed channel signal butejects the ASE and the residual clamping signal from chip 10. As shownin the embodiment of FIG. 19, no initial booster amplification of themultiplexed channel signals may be necessary so that, after channelsignal demultiplexing, the individual channel signals may be amplifiedvia SOAs 31(1) . . . 31(N) and the ASE and other optical noise removedby in-line filters 33(1) . . . 33(N) formed on chip 10. Alternatively,as shown in FIG. 19A, in a side view of RxPIC chip 10, output waveguides39 from AWG 30 include SOAs 20(N) which have a design essentially thesame as a PIN photodiode, such as the photodiode 140 in FIG. 8C. SOAs20(N) are inserted in each waveguide 39 to provide for channel signalamplification due such as to insertion loss. The amplified channelsignal then proceeds into a respective angled grating 183(N) whichfunctions as a narrow passband filter for reflecting the channel signalupwardly or transversely of PD 16. Grating 183(N) is transparent to theASE and other optical noise such as residual clamping lasing signal sothat these different wavelengths exit the chip as shown at 185.

Reference is now made to FIG. 20 illustrating a still further way ofrejecting ASE and the residual clamping signal from RxPIC chip 10. Inthis case, compared to the embodiment of FIG. 19A, the angled or blazedgrating filter 33A here is deployed on the input side of AWG 30 ratheron its output side where a single grating filter 39 is designed toreflect the bandwidth spectrum 37A of the multiplexed channel signals.The ASE and clamping signal are outside this spectrum and, therefore,are transparent to filter 39 and, as a result, are transferred throughthe filter and out of chip 10 as shown at 185. A heater 33B may beassociated with filter 39 to tune the bandwidth of the grating to bettermatch the wavelength spectrum of the channel signal grid to thewavelength grid of AWG 30.

FIG. 21 illustrates another embodiment for extraction of the ASE noisefrom chip 10 by employing a Mach-Zehnder interferometer (MZI) 41 in chip10 between GC-SOA 12 and AWG 30. Since spontaneous emission from GC-SOA12 is not coherent, such emission cannot be guided through MZI 41 and,therefore, functions as a filter for receiving only coherent channelsignal wavelengths. The residual clamping signal can be filtered by AWG30, as in the embodiment shown in FIG. 16 or by the employment of angledgrating filters 31(1) . . . 31(N) of FIGS. 17-19.

In FIG. 22, an additional AWG 43 is employed as a filter mechanism forASE and the residual gain clamping lasing signal. In this embodiment,the channel signals, λ₁ . . . λ_(N), are demiltiplexed via AWG 30 andpassed along output waveguides 39 through SOAs 31(1) . . . 31(N), tocover for insertion loss, to N×N AWG 43 which provides a narrow signalpassband rejecting any wavelengths outside the channel signal spectrum.The channel signals are then forwarded via waveguides 39A to theirrespective photodetectors 16(1) . . . 16(N) for optical to electricalsignal conversion.

In FIG. 33, the residual gain clamping signal is removed by forming inthe AWG input waveguide 37 from GC-SOA 12 a higher order angled orblazed grating 170 which deflects the higher or lower wavelength gainclamping signal, outside of the wavelength spectrum of the channelsignals, out of RxPIC chip 10. Grating 170 can be part of GC-SOA 12. Thefiltering out of the ASE in this embodiment would be accomplished in AWG30 as described in the embodiment of FIG. 16. It is important thatwaveguide 37 be single mode so that grating 170 functions to eject thegain clamping signal from chip 10.

FIG. 34 is similar to the embodiment of FIG. 33. In the plan view ofFIG. 34, InP-based RxPIC chip 10 comprises a GC-SOA 12 with an outputcoupled to a mode adapter 26 and a waveguide 182. Waveguide 182 includeshigher order grating 180 with an integrated heater 184 and PINphotodiode 17 positioned in the same planar level to receive lightreflected from grating 180. In the case here, the grating 180 has a peakwavelength that is substantially the same as the peak wavelength of theresidual gain clamping signal generated by GC-SOA 12. The residual gainclamping signal is, therefore, deflected out of waveguide to PINphotodiode 17 where it is detected and provides an electrical signaloff-chip to monitor the optical characteristics of the gain clampingsignal, such as, for example, its intensity and wavelength so thatadjustments can be made, if necessary, to the applied bias of GC-SOA 12.

FIG. 35 is similar to FIG. 34 except that it is a view perpendicular tothe view of FIG. 34 and illustrates, in cross-section, angled gratingfilter 180 in waveguide 182. A lateral waveguide 188 is provided in thesame as-grown layer as waveguide 182 to direct the residual gainclamping signal laterally to an etched trough or groove 187 formed inInP chip 10. Trough 187 has an angled surface at 45° with a depositedreflective surface 189 formed on the angled surface to reflect, alongits length, the gain clamping signal upwardly at 50 (or possiblydownwardly depending at what vertical position photodiode 17 isintegrated into chip 10) to an optical aligned photodiode 17 integratedin chip 10. Trough 187 may be etched by employing RIE. The space oftrough 187 may be filled with air or contain some others low refractiveindex medium.

Instead of the integrated photodiode 17 being directly vertical (FIG.35) or directly lateral (FIG. 34) of clamping signal filter 180,photodiode 17 can be offset transversely in chip 10 from the position ofgrating filter 180 as illustrated in FIG. 36. Also, instead ofphotodiode 17 being directly above or transversely of waveguide 182,photodiode 17 can be positioned below waveguide 182, as shown in FIG.37, and the reflected light from angled grating filter 180 is directeddownwardly at an angle, employing a second order or higher orderintegrated grating 183 to reflect the residual clamping signaldownwardly to integrated PIN photodiode 17. Such a second order grating183 can be in the same semiconductor layer in which filter 180 is formedor in a different or separate semiconductor layer.

A final approach for removing the residual gain clamping lasing signalis to employ this signal for pre-amplification of the oncoming channelsignals, provided that the gain clamping signal is also chosen to bewithin the absorption spectrum of the channel signals. This illustratedin FIG. 54 where the gain clamping is reflected back to mode adapter 26and propagates out of the front facet of chip 10, indicated as λ_(R),and into the fiber link to counter-propagate in the link. With λ_(R)designed to be within the absorption bandwidth of the incoming channelsignals, these signals will receive gain from its counter-propagation.Rather than a built-in reflector at the downstream end of GC-SOA 12, aquarter wavelength shift to the channel signal can be provided in thegrating of GC-SOA 12 or a multiple of that wavelength over 4N, so thatmost of the power of the gain clamp signal not utilized in the amplifierwill be directed out of the back or input port of chip 10 into the fiberlink.

In connection with the foregoing embodiment of FIG. 54, it is within thescope of this invention to provide on-chip laser pumps to provide forcounter-propagation of gain into optical link to provide for initialamplification of the incoming channel signals. This is illustrated inthe embodiments of FIGS. 44 and 45. In FIG. 44, an on-chip semiconductorRaman pump laser 230 is provided at one of the remaining first orderoutputs or higher order Brillouin zone outputs of AWG 30 to providecounter propagating signal, λ_(R), through AWG 30 and GC-SOA 12 into thefiber link to provide for pre-amplification of the incoming channelsignals. Such a pumping signal is transparent to the operation of GC-SOA12. Alternatively, as shown in FIG. 45, an on-chip semiconductor Ramanpump laser 232 could be coupled into AWG input waveguide 37 to providecounter propagating signal, λ_(R), through SOA 12A into the fiber linkto provide for pre-amplification of the incoming channel signals. Notehere, that an SOA 12A is denoted rather than a GC-SOA since the Ramanlasing pump laser signal can provide the on-chip gain clamping. SuchRaman pumps could be provided at both such locations if desired. Also,and importantly so, it should be noted that Raman pump laser 232,whether an on-chip semiconductor Raman laser or off-chip Raman fiberamplifier coupled in a waveguide 39 of AWG 30 or waveguide 37, can bedeployed instead of having an on-chip GC-SOA 12 or SOA 12A so that RxPICchip 10, in this embodiment, would be comprised of Raman pump laser 232,AWG 30 and photodetectors 16(1) . . . 16(N).

Reference is now made to FIG. 46, which discloses an on-chip signalmonitoring circuit and transmitter laser for providing a service channelsignal, λ_(S). Electro-optical circuit 234 is coupled to one of thefirst order outputs or higher order Brillouin zone outputs of AWG 30 andmonitors the channel signals, via AWG output 233, for their peakwavelength value to determine if the channel signals are on the peakwavelengths and, if not, to provide digitized information in servicechannel signal, λ_(S), back to a correspondent optical transmitter aboutthe quality of the channel signal wavelengths relative to thestandardized wavelength grid at the optical transmitter. As shownalternatively in FIG. 47, such an on-chip electro-optical circuit 236may also be provided with its input 235 into waveguide 37 tocounter-propagate service channel signal, λ_(S), through GC-SOA 12 andinto the optical link.

In connection with circuit 234 or 236 of FIGS. 46 and 47, reference isnow made to FIG. 64 illustrating an application of these types ofcircuits in an optical communication system. The system shown in FIG. 64comprises an optical transmitter PIC (TxPIC) chip 300 optically linkedin an optical point-to-point transmission system via optical link 288 toRxPIC chip 10. TxPIC 300 comprises a plurality of integrated componentsin plural paths to an AWG multiplexer 310 where each such path includesa DFB laser source 302, an electro-optical modulator 306 and a SOA 308(optional) coupled to an input of AWG 310. Each laser source 302 isoperated cw at a designated peak wavelength corresponding to astandardized grid, such as the ITU grid. The output of each laser source302 is modulated with an information signal at its respective modulator306. Modulator 306 may be, for example, a semiconductorelectroabsorption (EA) modulator or a Mach-Zehnder (MZ) modulator asknown in the art. The modulated signal may then be provided with gainvia SOA 308. SOAs 308 are optional and are preferred not to be anon-chip optical component because the overall power consumption of TxPICchip 300 will be less without them since most of the on-chip powerconsumption will come from the operation of SOAs 308. In the absence ofSOAs 308, DFB sources 302 will have to be operated at higher thresholdsand operating currents. The output of AWG multiplexer 310 is coupledoff-chip to optical link 288.

In order to operate TxPIC chip 300 in a stabilized manner, each DFBsource 302 is provided with a corresponding, integrated heater 304 andAWG 310 is provided with TEC 310A. A small sample of the multiplexedchannel signal output from AWG 310 is provided through a 1% tap, forexample, and is provided as an electrical signal input, via optical toelectrical domain conversion at on-chip PD 312, on line 311 toprogrammable logic controller (PLC) 316. PLC 316 discriminates among thedifferent channel signals, λ₁ . . . λ_(N), to determine if the operatingwavelengths of DFB sources are at their desired wavelength peaks asdetermined by reference to a peak wavelength reference memory. Thisdiscrimination process can be carried out by employing dithering signalson the modulated channel signals providing each such signal with anidentification tag. As a result, each of the channel signals can beseparated and analyzed as to its wavelength to determine if it is at aproper wavelength relative to a standardized grid, such as the ITU grid.Such a discrimination scheme is disclosed in U.S. patent applicationSer. No. 10/267,331, filed Oct. 8, 2002 and entitled TRANSMITTERPHOTONIC INTEGRATED CIRCUITS (TxPIC) AND OPTICAL TRANSPORT NETWORKSEMPLOYING TxPICs, which application is owned by the assignee herein andis incorporated herein by its reference. If the peak wavelength of anyparticular DFB laser source 302 is off, its operating wavelength iscorrected to the desired peak grid wavelength by a signal provided fromPLC 316 to heater control circuit (HCC) 320 which provides a temperaturecontrol signal to a corresponding laser source heater 304 for increasingor decreasing the operating temperature of its DFB laser source 302 byan amount necessary to increase or decrease its operating wavelength tobe substantially the same as desired and stored peak wavelength.

The temperatures of DFB laser sources 302 are not monitored but thetemperature of AWG is monitored with a thermistor 313 which provides PLC316 current information of the AWG ambient temperature via input 315.PLC 316 can then provide a control signal to heater control circuit(HCC) 318 to provide a temperature control signal to TEC 310A toincrease or decrease the ambient temperature of AWG 310. In this mannerthe wavelength passband grid of AWG 310 may be shifted and adjusted tooptimize it to be as close as possible to the standardized grid and thewavelength grid of DFB laser sources 304.

Also, the input side of AWG 310 includes a port 317 relative to a higherorder Brillouin zone of the input side of AWG 310 for the purpose ofreceiving a service signal, λ_(s), from RxPIC 10 via optical link 288,which is explained in further detail below. This service signal isdemultiplexed by AWG 310 and provided on port 317 as an output signaland thence converted to the electrical domain by integrated, on-chip PD314. The electrical signal from PD 314 is taken off-chip and provided asan input 319 to PLC 316.

At RxPIC chip 10, AWG demultiplexer 30 includes higher order Brillouinzone outputs 289A and 289B to receive respective channel signals, suchas, for example, λ₁ and λ₂ or any other such signal pairs, in order todetermine if their grid wavelengths are off the desired peak wavelengthand, if so, by how much. Also, using these two channel signals as awavelength grid sample, a determination can be made as to whether theAWG wavelength is shifted and, if so, by how much. Photodetectors 290Aand 290B provide an electrical response to optical signals on outputs289A and 289B which signals are provided on lines 291A and 291B to PLC292. These PDs 290A and 290B are sensitive to the peak optical responsesof these-signal outputs and can be deployed in the electrical domain todetermine if their peak wavelengths are off a desired peak wavelength.Also, if the delta shift, δ, of both is approximately same amount and inthe same direction (both either a red shift or a blue shift relative totheir desired wavelength peak), this delta shift is indicative that ashift in the wavelength grid of either Rx AWG 30 or possibly Tx AWG 306has occurred. In these cases, PLC 292 can first make adjustment to theRX AWG grid via heater control circuit (HCC) 294 via line 295 to Rx TEC30A to either increase or decrease the ambient operating temperature ofAWG 30 to shift its wavelength grid either to the longer or shorterwavelength side based on the determined delta shift. If this adjustmentdoes not resolve the issue, then data relating to either the DFB channelsignal wavelengths or the Tx AWG wavelength gird being offset from itsdesired setting can be forwarded over optical link 288 as a servicechannel signal, λ_(s), for correction at the transmitter end. In thesecircumstances, PLC 292 can forward such wavelength and grid correctiondata as a service channel signal, λ_(s), via an electrical correctiondata signal on output line 293 to service signal modulator 294, whichmay be comprised of an on-chip integrated DFB laser and EO modulator, toprovide this signal through AWG 30 and counter propagation via opticallink 288 to TxPIC 300. This service channel signal, Xs, is thendemultiplexed via AWG 310 and provided on higher order output 317 to PD314. The electrically converted service signal data is deciphered by PLC316 which makes a correction to the operating wavelength of a DFB lasersource 302 via HCC 320 and/or makes a correction to the wavelength gridof AWG 310 via HCC 318.

Reference is now made to FIG. 69 where the RxPIC chip 10 and the TxPICchip 300 of FIG. 64 are deployed as an optical-to-electrical-to-optical(OEO) converter 400 for optical signal regeneration in an opticaltransmission link. As shown in FIG. 69, the incoming multiplexed channelsignals, λ₁ . . . λ_(N), are received by RxPIC chip 10 from optical link408, demultiplexed and converted into corresponding electrical channelsignals and provided through low impedance coupling lines 403 toelectronic regenerator 401 comprising a plurality of circuits 402, 404and 406, which may be comprised of a chip set for each demultiplexedelectrical channel signal received from RxPIC 10. Circuit 402 comprisesa transimpedance amplifier and a limiting amplifier. Circuit 404comprises retiming and reshaping circuit where the bit clock isextracted from the signal to reclock the signal and regenerate thechannel signal. The regenerated electrical channel signal may be furtheramplified via circuit 406 and provided as an output on low impedancelines 407 to the electro-optical modulators 306 of TxPIC chip 300 wherethe corresponding optical signals are again regenerated, the pluralchannels are multiplexed via AWG 310 and provided as multiplexed channelsignal, λ₁ . . . λ_(N), on optical link 410. OEO converter 400 has theadvantage of being cost effective, compact, easily field-replaceablecompared to previous OEO converters and eliminates of the problems ofoptical-to-optical converters comprising erbium doped fiber amplifiers(EDFAs), functioning as line amplifiers, that need continuous attentionrelative to saturation where if the input signal power increases ordecreases, the amplifier gain drops or increases. Also there is aproblem of gain nonflatness across the channel signal wavelength band sothat gain equalization techniques need to be provided. Also, theseoptical line amplifiers, while being the choice today for transmissionline optical signal amplification, they are not as compact or readilyreplaceable as converter 400 particular when changes are made toincrease the number of multiplexed channels and traffic to carried overan optical link. OEO converter 400 of FIG. 69 eliminates this problemsand considerations particularly since the converter can be easilyswapped in between optical links 408 and 410 with a converter havinglarger channel capacity.

An important feature of RxPIC chip 10 is the monolithic incorporation ofoptical components, in particular GC-SOA 12 and AWG 30, which canprovide polarization independent gain to the channel signals andfunction as a polarization insensitive waveguide grating router ordemultiplexer. In GC-SOA 12, either the stable lasing in the TE mode orTM mode to provide the DFB clamping signal is preferred. It should bestable over the life of GC-SOA 12 or chip 10, i.e., discriminationbetween the TE mode and the TM mode should be made large. If both TE andTM modes lase or alternate between lasing in these polarization modes,this will lead to unstable operation of GC-SOA 12. This can alsomanifest itself in gain variation and additional noise in GC-SOA 12.

Relative to GC-SOA 12, one way of accomplishing polarization independentgain as incorporated in a PIC is to provide for the active region, suchas active region 132 in GC-SOA in FIG. 8A, to have alternately strainedtensile and compressive multiple quantum wells of Q (InGaAsP) to balancethe polarization dependent gain across the plural wells. Thus, if sixsuch wells are utilized in active region 132, three wells are tensilestrained and three wells are compressively strained and the former arealternated with the latter. In this regard, see the article of M. A.Newkirk et al., entitled, “1.5 mm Multiquantum-Well SemiconductorOptical Amplifier with Tensile and Compressively Strained Wells forPolarization-Independent Gain”, IEEE Photonics Technology Letters, Vol.4(4), pp. 406-408, April, 1993, which article is incorporated herein byits reference. Another approach is to potentially utilize the techniquesuggested in U.S. Pat. No. 5,790,302, which patent is incorporatedherein by reference, where a two part grating would be utilized whichhas a minimum reflection at a first wavelength, which is also minimumfor TE portion of the light, and a minimum reflection at a secondwavelength, which is also minimum for TM portion of the light, and aproduct of these reflections is a minimum for both wavelengths andoptimized at an intermediate wavelength so that the resulting TE and TMmodes will be substantially the same.

A further way of rendering GC-SOA 12 polarization insensitive, which isshown in FIGS. 38 and 39, is to employ a λ/4 grating, shown at 171 inFIG. 38, or two λ/8 gratings, shown at 173 in FIG. 39, in the gainclamping grating of GC-SOA 12. These gratings can suppress the strongerof the two TE and TM modes to render them more substantially the same.Another way, illustrated in FIG. 40A, is to employ a loss refractiveindex grating to enable one of the two modes over the other mode. Thisis accomplished by making the grating layer 128 to have a bandgapsimilar to that of active region 132 so that the grating will functionas a selective loss for one of the modes thereby enhancing the othermode. On the other hand, by making the grating layer 128 to have asignificantly larger bandgap compared to that of active region 132, thenthe grating will function to be index selective of one of the modeswhile suppressing the other mode. A further approach is to perturb theactive region 132 itself employing a grating 128A, as shown in FIG. 40B,to provide selective gain for one of the modes.

A still further way of ridding or otherwise suppressing on of thepolarization modes in the GC-SOA 12 is by employment of an AR coating asillustrated in FIG. 41. The AR coatings applied to the input and outputregions of GC-SOA can be designed to favor one polarization mode overthe other since these AR coatings 190 are typically broadband. Thisembodiment would be best utilized where GC-SOA 12 and AWG 30 areseparate, discrete optical components.

Another way of ridding or otherwise suppressing one of the polarizationmodes in the GC-SOA 12 is through the employment of a grating shape thatis tailored to favor one polarization mode over another. As shown inFIG. 42, this is illustrated as a square wave form 192 but it could bespecifically tailored via other grating shapes, such as triangular or awaveform similar to sinusoidal, to accomplish the same result.

Also, as illustrated in FIG. 43, the grating in the grating layer 191could be made to be a second or higher order grating where the plane ofscattering is relaxed for one of the polarization modes, that is, β_(TE)is not equal to β_(TM). With the first order grating, the direction ofmode scattering is in the plane of the waveguide. However, with a secondor higher order grating, one of the modes, shown at 194, can bepreferentially scattered transversely out of waveguide 193 while theother mode 196 remains in waveguide 193.

Lastly, a frequency selective feedback optical element that selectsbetween the TE mode and the TM mode can be utilized in the RxPIC chip 10as incorporated in waveguide 37 between GC-SOA 12 and AWG 30.

With respect to polarization insensitivity at AWG 30, reference is madeto FIG. 23 where AWG 30 comprises at least one input waveguide 37 and aplurality of output waveguides 39 between which are space regions 32 and36 and N grating arms 34. Also, shown are higher order Brillouin zoneinput arms 37A and higher order Brillouin zone output arms 39A. As shownin the cross-section in FIG. 8C of an AWG waveguide 34, the fabricationprocess generally lends itself to geometrically forming rectilinearshaped cross-sectional waveguide structures. Thus, the TM and TE modesof the signals will be favored one over the other. Only way of solvingthis problem is illustrated in U.S. Pat. No. 5,623,571, which isincorporated herein by its reference. As is known, the TM mode willpropagate faster through grating arms 34 than the TE mode. What can bedone is slow down the propagation of the TM to equal, in phase, thepropagation of the TE mode. A patch 30E is made in the overlyingcladding layer or top glass layer over the waveguide, i.e., some of theoverlying layer is removed in patch region 30E to increase thebirefringence in region 30E relative to the birefringence remaining inother overlying regions of the same grating arms 34. As a result, abalance can be achieved in the propagation phase between the TE and TMmodes so that an in-phase relationship between these modes is maintaineddependent on the path lengths of the arms in region 30E versus thoseportions outside of this region for the same arrayed arms. One way ofdetermining the extent and depth of patch 30E is providing a separatewavelength, λ_(XT), in higher order inputs 37A and monitor thosewavelengths at higher order outputs 39A to look at the polarizationcharacteristics of the these signals to determine what depth must beetch for patch region 30E to appropriately change the TE/TM ratio andachieve polarization independence of AWG 30.

Another approach to achieve polarization insensitivity in AWG 30 is toprovide a fabrication technique that provides for nearly squarecross-sectional arrayed waveguides so that AWG 30 will havesubstantially zero birefringence waveguides. This is described andtaught in the articles of J. Sarathy et al., entitled, “PolarizationInsensitive Waveguide Grating Routers in InP”, IEEE Photonics TechnologyLetters, Vol. 10(12), pp. 1763-1765, December, 1998, and in J. B. D.Soole et al., entitled, “Polarization-Independent InP Arrayed WaveguideFilter Using Square Cross-Section Waveguides”, ELECTRONIC LETTERS, Vol.32(4), pp. 323-324, Feb. 15, 1996, both of which are incorporated hereinby their reference.

Reference is now made to FIG. 24 which illustrates another approach forachieving polarization insensitivity through the incorporation of SOAs45(1) . . . 45(N) in each of the arms 34 of AWG 30. As is known, the TMmode will propagate faster through grating arms than the TE mode. Whatcan be done is slow down the propagation of the TM to equal in phase thepropagation of the TE mode. This was done in FIG. 23 by using patch 30E.Here, effectively it is accomplished by using SOAs which are ofdifferent lengths, so that the TM modes in arms 34 will be attenuated.The amount of required attenuation can be calculated through AWGcomputer simulation of the individual wavelength channels so that thelength of SOAs 45 can be determined and attenuation of the TM mode overthe TE mode can be provided for their equalization in arrayed arms 34.In this case, see, for example, the article of M. Zingibl et al.,entitled “Planarization Independent 8×8 Waveguide Grating Multiplexer onInP”, ELECTRONICS LETTERS, Vol. 29(2), pp. 201-202, Jan. 21, 1993 andpublished European patent application EP 0731576A2, dated Sep. 11, 1996,both of which are incorporated herein by their reference. Thus, arms 34can be provided to polarization insensitive as well as provide forequalization of signal gain across the wavelength grid of AWG 30.

Reference is now made to several embodiments relating to architecturefor coupling the electrical signal outputs from bonding pads 28 of theRxPIC chip photodiodes 16 to a RF submount substrate or a miniaturecircuit board or a monolithic microwave integrated circuit (MMIC), withparticular reference being made to FIGS. 28, 29, 30, 51 and 55. In FIG.28, a side view of a particular configuration for RxPIC chip 10 is shownin a schematic form showing in integrated form GC-SOA 12, AWG 30 andplural photodetectors or PINs 16(1) . . . 16(N). Output pads 28(1) . . .28(N) of PINs 16(1) . . . 16(N) are solder bumped to output pads 154(1). . . 154(N) of respective transimpedance amplifiers (TIAs) 152(1) . . .152(N) formed on NMIC 150. TIAs 152 provide for conversion of thecurrent signals developed by the respective PINs 28 into voltagesignals. MMIC 150 is also shown here to include a portion 153 of RFsubmount 150 which includes other circuit components as known in theart, such as an automatic gain control (AGC) circuit for increasing thesignal strength and range, which circuit can apply a gain control signalto TIAs 152 or provide the signal across the differential input of TIAs152; a power amplifier (PA) to increase the signal gain from TIAs 152;and a clock and data recover (CDR) circuit. A CDR circuit (not shown)recovers the embedded clock from a baseband non-return-to-zero (NRZ) orreturn-to-zero (RZ) data stream and generates a clean data stream (e.g.,data that does not have timing jitter due to, for example, the limitedbandwidth of the transmission channel). The clock recovery function of aCDR circuit is typically performed with a phase-locked loop (PLL) whichrequires a tunable clock signal, such as generated by a voltagecontrolled oscillator (VCO). This arrangement provides for compactnesswith RF submount 150, carrying receiver electronics in overlyingrelation to PIC chip 10 and supported at a bonding point of bonding pads28 and 154. As a result, RF board 150 is spatially supported above chip10 to provide for a space between them for circulation of air andcooling.

FIG. 55 is substantially the same as FIG. 28 but a more detailed versionof FIG. 28. As shown in FIG. 55, an optical link is coupled to the inputend of PIC chip 10, where the received multiplexed signals, λ₁ . . .λ_(N), are provided to GC-SOA 12 for amplification and thence viawaveguide 37 provided to AWG 30 where the signals are demultiplexed andprovided on output waveguides 39 to PIN photodetector array 16(1) . . .16(N). The electrical signal outputs from these photodetectors isprovided to PIN contact 28(1) . . . 28(N) which are then solder bondedto corresponding TIA contacts 154(1) . . . 154(N) providing electricalconnection to corresponding TIAs 160(1) . . . 160(N) and thence to poweror limiting amplifiers 162(1) . . . 162(N). The outputs of poweramplifiers 162 may be provided to other circuit components such as CDRcircuits or the electrical signals can be taken off of RF submount 150via RF transmission lines 163 at bonding pads 163A.

With reference to FIG. 28A, the arrangement is shown where PIC chip 10comprises input GC-SOA 12 and AWG demultiplexer 130. The multiplexedsignal outputs, λ₁ . . . λ_(N), are mirrored off of chip 10 by 45°mirror 155. Note that, in this embodiment, the PIN photodiodes 16(1) . .. 16(N) are formed on RF submount 150 rather then on chip 10. Theoptical signals reflected from mirror 155 are directed up to the alignedarray of PINs 16(1) . . . 16(N) where the converted electrical signalsare directed to corresponding TIAs, PAs and CDR circuits on RF submount150. Also, a lens array on a separate board can be employed betweensubmount 150 and chip 10 to aid in focusing the signals, λ₁ . . . λ_(N),onto the top detection surface PIN photodiodes 16(1) . . . 16(N) on RFsubmount 150, such as in a manner illustrated in the article of A. E.Stevens et al., entitled, “Characterization of a 16-ChannelOptical/Electronic Selector for Fast Packet-Switched WDMA Networks”,IEEE Photonics Technology Letters, Vol. 6(8), pp. 971-974, August, 1994,which article is incorporated herein by its reference. RF submount 150is secured to chip 10 by means of solder ball bonding via solder balls156.

Reference is now made to FIG. 28B which illustrates a further example ofan arrangement of chip 10 and RF submount 150. In this arrangement,additional boards are employed comprising submount 166 and filler board164. Only one end of InP chip 10 is shown that includes AWGdemultiplexer 30. A 45° angled edge 157 is formed along the output edgeof output waveguides 39 from AWG 30 handling demultiplexed channelsignals, λ₁ . . . λ_(N). The angled edge 157 is coated with a mirrorsurface as is known in the art. As in the case of the embodiment of FIG.28A, RF submount includes PIN photodetectors 16(1) . . . 16(N) which aresurface photodetectors aligned with the respective signals, λ₁ . . .λ_(N), where the signals are then processed via the TIAs, PAs 162(1) . .. 162(N) and CDR circuits on RF submount 150. Filler board 164 and RFsubmount 150 provide support for PIC chip 10, as secured via ballbonding 156, and both filler board 164 and RF submount 150 are supportedon submount 166.

Reference is now made to FIGS. 29 and 30 which disclose wire bondedversions of connecting photodetector pads 28 to RF submount pads 159 onone or two RF circuit boards. All the electronic RF circuit componentsare on microwave submounts 150, 150A and 150B as it is easier to controlthe circuit impedance on these circuit submounts rather than on PIC chip10. Microwave submounts 150, 150A and 150B may be, for example, ceramicsubmounts. As shown in FIG. 29, photodetector pads 28 are wire bonded tocorresponding bonding pads 159 on microwave submount 150 and the signalsare then feed into TIAs 160 and thence on to other circuit components,as previously described, via high speed transmission lines 168.

As shown in FIG. 30, in order to save space and provide for morecompactness, photodetector bonding pads 28 may be placed in two or morestaggered rows on the edge of PIC chip 10 and one or more pad rows ofpads 28A are wire bonded to TIA bonding pads 159A on a first RF submount150A which are correspondingly coupled to TIA circuits 160A. Theremaining pad row or rows of pads 28B are wire bonded to TIA bondingpads 159B on a second RF submount 150B which are correspondingly coupledto TIA circuits 160B. By staggering the pad rows on chip 10 as well asemploying more than one RF submount, short bonding wires can be employedso that the inductance relative to the microwave circuits can beminimized. In this connection, high speed transmission boards 150A and150B are mounted in spaced relation to RxPIC chip 10 by a spacingdistance greater than 5 μm. Also, the staggered spacing of pads 28 stillallows for good separation between the accompanying photodetectors 16while permitting the shrinkage of the overall PIC dimensions,particularly if the staggered rows are provided along two sides of thePIC chip 10, as demonstrated for one row of pads 28 shown in FIG. 6. Forexample, both the diagonal spacing and the side-by-side spacing of pads28A and 28B on chip 10 may be a minimum of about 250 μm. This spacing iscritical to insure minimal crosstalk between channel signals.

Reference is now made to FIGS. 48-51. In FIG. 48, the simpletransimpedance amplifier (TIA) 200 with a feedback resistor 201 is shownand is well known in the art. The gain of amplifier 200 is dependentupon the input signal level and the signal current, I_(s), can vary overa fairly large range. An automatic gain control (AGC) circuit can becoupled to amplifier 200 to linearly control its gain.

As shown in FIG. 49, the transimpedance amplifier (TIA) 202 can have adifferential output. The differential output helps to reduce the noiseat the output of the amplifier. However, the employment of a trulydifferential input, as illustrated in FIG. 50, can significantly reduceany cross-talk on RxPIC 10 between the channel signals output. For thisscheme to work, a photodiode reference input is also provided to TIA202. As shown in FIG. 50, TIA 204 has a differential input as well asdifferential output. Feedback resistors 205 are provided for bothdifferential inputs to TIA 204 as well as DC blocking capacitors 203 areprovided in these inputs too. The differential outputs of TIA 204 arecoupled to limiting amplifier 206. As shown in FIG. 50, one of thedifferential inputs is coupled to a respective signal photodetector 16and the other differential input is coupled to a respective referencephotodetector 206 which is also formed in RxPIC chip 10 as shown in FIG.51, to be discussed next. In the case here, both photodetectors 16 and206 will substantially detect the same noise environment, i.e.,photodetector 16 will detect the signal plus noise and photodetector 206will detect the noise scattered in chip 10, which is generally crosstalknoise and optical noise from other optical components integrated in thechip. As a result, the crosstalk and noise can be substantiallycancelled out via the differential input to TIA 204.

As shown in FIG. 51, photodetectors 16(1) . . . 16(N), for detecting thechannel signals via waveguides 39 are formed along an edge of chip 10and are primary photodetectors PD_(P1), PD_(P2), etc. Companionphotodetectors 206(1) . . . 206(N), for detecting optical noise, inparticular crosstalk noise, are alternating with photodetectors 16(1) .. . 16(N) and are secondary photodetectors PD_(S1), PD_(S2), etc. Also,ground pads 208 on chip 10 are connected via bonding wires to groundpads 214 on the RF submount 150. Photodetectors 16 and 206 are separateda sufficient amount to prevent undue crosstalk, e.g., about 250 μm ormore.

Alternatively, one or less than all of the entire companionphotodetectors 206 can be deployed on RxPIC chip 10 to provide anoptical noise signal for all or more than one primary photodetector16(N), rather than providing one secondary photodetector 206(N) inproximity to each and every primary photodetector 16(N). Although thiswill function to establish a useful noise floor for the detected RxPICchannel signals for use with differential TIAs 204, the establishedsignal noise floor will not be as accurate as in the case where there isone companion photodetector 206 for every primary photodetector 16 sincethe amount of optical noise at every primary photodetector location onthe chip will not necessarily be the same.

It should be noted that in FIG. 51, bonding wires 216 should be as shortas possible to reduce photodetector inductance. Another way ofeliminating this inductance is to eliminate these bonding wires 216 alltogether by forming TIAs 204 directly on chip 10. This is accomplishedby utilizing InP-HBT or InP-HEMT circuitry on chip 10.

It is desirable that certain components be included with chips 10 orformed in an InP wafer with the chip die configuration to provide fortesting capabilities. Some examples are shown in connection with FIGS.52 and 53 to be described now.

In FIG. 52, higher order Brillouin zone arms 39A and 39B are taken offof output space region 36 of AWG 30 and angled facets 220 are providedin a manner similar to facet 225 shown in FIG. 53, to be next described,so that the signal light of one or more channels can be taken off chipwhile the chip is still in the InP wafer. The light input is provided toAWG 30 in a manner as shown in FIG. 53. Thus, the optical output fromthe angled on-chip reflectors can be detected employing an opticalinterrogation probe where the output is collected and analyzed todetermine if the signals are being properly demultiplexed through AWG 30relative to the AWG wavelength grid, the relative intensity of thesignals, their peak frequency, etc. This approach saves a great deal oftime and expense by eliminating wafers with poor quality opticalcomponents without going to the expense of cleaving the wafer intoindividual die and testing them separately. Alternatively, instead of anangle facet 220, a higher order grating can be formed at this point todeflect the signal out of the chip for detection. Also, instead ofeither an angled facet 220 or a higher order grating, photodetector orphotodetectors 222 can be fabricated directly on chip and employed totest the channel signal properties while the chip die remain in-wafer.These same photodetectors 222 can be employed later, after the remove ofchip from the wafer for signal monitoring and feedback indicative of theoperating wavelength peaks of the channel signals and the amount, ifany, that they are off relative to a predetermined wavelength grid, suchas standard ITU grid.

It should be pointed out that, in connection with FIG. 52,photodetectors 16(1) . . . 16(N) of an as-cleaved chip can be initiallyemployed to examine the total signal and differential signal betweendetectors to tune the wavelength grid of AWG 30 via TEC 30A. Thetemperature of AWG 30 is changed so that its wavelength grid bestmatches the wavelength grid of the channel signals to be demultiplexed.In order to accomplish this grid tuning, it is preferred that at leasttwo of the channel signals need to be detected. Once the AWG wavelengthgrid has been optimized to a standardized grid, such as the ITU grid,the factory setting for TEC 30A is placed in memory of the RxPICcontroller circuitry. Such circuitry is beyond the scope of thisdisclosure and will be detailed in later applications. More is saidabout this monitoring and adjustment in connection with FIGS. 73, 73Aand 73B.

Reference is now made to FIG. 53 illustrating the in-wafer testing ofRxPIC chips 10. As mentioned previously, it is advantageous to test theRxPIC chips 10 in-wafer because if the initial testing of a majority ofthem results in poor performance, time and expense of dicing the chipsfrom the wafer as well as subsequent individual chip testing has beencircumvented. Also, such in-wafer testing can be easily automated sincethe input to each chip is at a known or predetermined location in theInP wafer. As a result, in-wafer testing can be handled in a matter ofseconds to a few minutes where individual die testing would take aperiod of days and, therefore, lead to higher product costs. In FIG. 53,the in-wafer chips or die 10 include an in-wafer chip sacrificialspacing or region 224. Within region 224, there is formed an in-wafergroove 223 having an angular mirror surface 225A formed via selectiveetching. A mirror coating, which is optional, could be also deposited onthese surfaces. The angled surfaces 225 are preferably angled at 45° sothat an interrogation beam, normal to the surface of the wafer, may bemoved by an automated mechanism over the wafer to surface 225 to providean optical signal input, such as a plurality of test pulse channeloptical signals into chip 10 via its chip input. The testing isaccomplished by probe testing the outputs of the respective photodiodes16. Alternatively, it should be noted that that grooves 223 also servedat the opposite end of the in-wafer RxPIC chip 10 serve as a point todetect the rear end light from photodetectors 16 through an opticalpickup such as an optical fiber coupled to an off-chip photodetector.The testing of an on-chip GC-SOA 12 and AWG 30 is conducted as indicatedrelative to the discussion of FIG. 52 or using the probe card asdisclosed in U.S. patent application, Ser. No. 10/267,331, supra, whichapplication has been previously incorporated herein by reference.Characteristics that may be checked, for example, include optical powerof the gain clamped-SOA or passband response and insertion loss of theAWG. After testing is complete, the region 224 can be cleaved away fromchip 10 as indicated by the set of cleave lines in FIG. 53.

Reference is now made to FIGS. 57 and 58. Active region 240 of GC-SOA 12may be a multiple quantum well (MQW) region as illustrated in FIG. 57 ormay be a single active layer, such as illustrated in FIG. 58. In FIG.57, the MQWs 242 are compressively strained and the barriers 244 may betensile strained to produce an overall strain-balanced structure inwhich the electron light hole, mostly TM barrier transition is slightlyfavored over the larger energy electron heavy hole TE well transition.Alternatively, the MQWs 242 can be alternately tensile and compressivelystrained. The TE:TM emission ratio can be adjusted by the amount ofstrain and the number of compressive versus tensile barriers havingidentical effective bandgaps. Such treatment provides for a polarizationinsensitive SOA. See Chapter 5, “Semiconductor Laser Growth andFabrication Technology”, Section IV, “Polarization InsensitiveAmplifiers by Means of Strain”, pp. 177-179, in the book entitled,“Optical Fiber Communication—IIIB” (Vol. 2), edited by Kaminow and Koch,Academic Press, published in 1997, which Section is incorporated hereinby its reference. For further background, see the articles of M. A.Newkirk et al., entitled, “1.5 mm Multiquantum-Well SemiconductorOptical Amplifier with tensile and compressively Strained Wells forPolarization-independent gain”, IEEE Photonics Technology Letters, Vol.4(4), pp. 406-408, April, 1993, and of Young-Sang Cho et al, entitled,“Analysis and Optimization of Polarization-Insensitive SemiconductorOptical Amplifiers with Delta-Strained Quantum Wells”, IEEE Journal ofQuantum Electronics, Vol. 37(4), pp. 574-579, April, 2001, both of whichare incorporated herein by their reference.

In FIG. 58, if a single active layer is employed, the Q active layer 246may be tensile strained and the confinement layers 248 may becompressively strained (the latter is optional).

AWG 30 can be made substantially temperature insensitive and itswavelength spectrum stabilized over time so that changes in the ambientdo not affect changes in the AWG arm lengths thereby changing its narrowpassband characteristics. This T-insensitivity can be achieved in aInGaAsP/InP AWG 30 by employing a high dn/dT array waveguide 1.3Q region260 and two low dn/dT arrayed waveguide 1.1.Q regions 262 as illustratedin FIG. 59 and described in the article of H. Tanobe et al., entitled,“Temperature Insensitive Arrayed Waveguide Gratings in InP Substrates”,IEEE Photonics Technology Letters, Vol. 10(2), pp. 235-237, February,1998, which article is incorporated herein by its reference. As taughtin this article, the difference in the optical path length of any pairsof waveguide arms 34 in the array becomes longer when the waveguides areheated. This brings about a red shift to the demultiplexed channelwavelength toward a longer wavelength. This effect is cancelled out byemploying 1.1Q and 1.3Q regions 260 and 262 providing different sectionswith different values for dn/dT in the waveguides dependent on apredetermined relationship between their lengths. Alternatively, asshown in FIG. 60, a heater or electrically pumped region 264 can beprovided over a portion of wavelength arms 34, operated via atemperature controller, to stabilize the wavelength spectrum of AWG 30by changing the refractive index of the arms to compensate forwavelength changes from desired peak wavelength passbands of arms 34.Region 264 can also include one or more regions that are provided with amaterial or materials overlying AWG 30 where the materials havedifferent coefficient of thermal expansions, such as decreasing orincreasing in length with increasing temperature, to provide a moreathermal structure. This can also be accomplished by using heater stripsor current pumping stripes 266, as shown in FIG. 61, where the appliedbias to strips 266 can be uniform or non-uniform across the strip array,e.g., applied in a monotonically increasing to monotonically decreasingmanner across the array. Thus, the applied bias can be selectivelyand/or independently varied to compensate for temperature driven indexchanges of the AWG.

Reference is now made to FIG. 70 illustrating a forward error correction(FEC) enhanced optical transport network 500 shown as single directiontransmission in a point-to-point optical transmission link including atleast one TxPIC 502 and at least one RxPIC 504 optically coupled byoptical link 506. It should be understood that network 500 can bebidirectional where TxPIC 502 can also be a transceiver and including areceiver, such as RxPIC 504, which transceivers are also illustrated inpatent application Ser. No. 10/267,331, filed Oct. 8, 2002, which isincorporated herein. In such a bidirectional network, optical link 506would be deployed for use with the eastbound and westbound traffic ondifferent channel wavelengths. TxPIC 502 includes a plurality of DFBlaser sources 508(1) . . . 508(N) optically coupled, respectively, toelectro-optic modulators (MODs) 510(1) . . . 510(N), in particular,electro-absorption modulators or Mach-Zehnder modulators. The outputs ofthe modulators 510(1) . . . 510(N) are optically coupled to an opticalcombiner or MUX 511, e.g., an arrayed waveguide grating (AWG). As shownin FIG. 70, each DFB laser source 508 has a driver circuit 512(1) . . .512(N). Each modulator 510(1) . . . 510(N) includes a driver 514(1) . .. 514(N) for input of the bias point of the modulator and the datastream for modulation. FEC₁ . . . FEC_(N) encoders 518(1) . . . 518(N)are used to reduce errors in transmission of data transmitted overnetwork 500. These encoders may also be a joint FEC encoder 520 tojointly encode bit code representative of transmitted data. As wellknown in the art, the performance of a received data signal is measureddeploying an eye diagram, such as shown in FIG. 71A, which will bediscussed in more detail later. Further, FEC₁ . . . FEC_(N) encoders518(1) . . . 518(N) are deployed to reduce the bit error rate (BER) bytransmitting on the laser source light output with additional bitsthrough the employment of error-correcting code containing redundantinformation of the data bit stream, along with the transmission of themain data bits. The error-correcting code is deployed at the opticalreceiver for correcting most errors occurring in transmission of thedata bits thereby increasing the immunity of system 500 from noiseresulting in reduced channel crosstalk. The encoders 518 are shown inconnection with the transmission of redundant encoded data bits at DFBlaser sources 508 but this redundant code can also be transmitted atmodulators 510, i.e., the FEC encoders 518 can be deployed between themodulated data source or modulators 510 and the multiplexer 511. At theoptical receiver end, the RxPIC chip 504 comprises a demux ordemultiplexer 522 and a plurality of photodiodes (PDs) 523(1) . . .523(N), one each for each channel signal λ₁ . . . λ_(N) transmitted fromTxPIC chip 502. RxPIC chip may also include an optical amplifier at itsinput, either integrated into the input of the chip, e.g., again-clamped SOA (not shown), or an external optical amplifier, e.g., anEDFA (not shown). The electrically converted signals are respectivelyreceived in receivers 524(1) . . . 524(N) and the FEC encoded date isdecoded at FEC₁ . . . FEC_(N) decoders 526(1) . . . 526(N). The detailsrelative receivers 524 are shown in FIG. 71 and will be discussed infurther detail later. It should be noted that FEC₁ . . . FEC_(N)decoders 526(1) . . . 526(N) may also be a joint FEC decoder 530 for allsignal channels which are decoded as a group.

As shown in FIG. 70, the FEC decoded data is received on line 527 forproviding the BER data to real-time BER controller 528. Controller 528discerns such parameters such as output power level of DFB laser sources508, the bias point and chirp of modulators 510 and decision thresholdvalues, such as the threshold decision voltage at the receiver as wellas the phase and threshold offset relative to the eye diagram.Controller 528 provides feedback service channel information via line531 feedback through demultiplexer 522, optical link 506 (as shown atarrow 526) to controller 516 on the transmitter side. The servicechannel data is then provided to DFB laser sources 508 via line 513drivers 512 to correct the intensity of a respective laser source. Also,correction signal are provided on line 509 to modulator drivers 514 ofmodulators 510 to make adjustments on the rise time of modulated data,particularly in cases of lone data pulse “1's”, to adjust the cross-overpoint to be further discussed in connection with FIG. 71B, to adjust thebias point of the modulator and to enhance the extinsion ratio of themodulator as well as change the modulator chirp (alpha parameter), andto make RF magnitude adjustments on the modulated data stream. At thereceiver, controller 528 also makes adjustments to receivers 524 inparticular to adjust decision threshold values such as phase andthreshold offset to respectively to achieve better data recovery withinthe decision window of the eye and offset the decision threshold fromthe eye position of most noise on the data stream.

Reference is now made to FIG. 71 which is a detail of one signal channelin the optical receiver comprising RxPIC chip 504 and receiver 524. Chip504 includes photodiodes 523 from which an electrical data signal isobtained and pre-amplified by transimpedance amplifier and mainamplifier by automatic gain control amplifier shown together at 540. Theamplified signal is then provided to electronic dispersion equalization(EDE) circuit 542 followed by clock and data recovery (CDR) circuit 544after which the data is passed on SerDes circuit 546 which is aserializer/deserializer circuit, as know in the art, for converting theserial data into parallel format for faster handing of the data stream.CDR circuit 544 may be part of the SerDes circuit 546. At this point,the trailing overhead, following the client payload, which carries theFEC coded data format is decoded at FEC decoder 526 and may be providedas feedback on line 552 to EDE circuit 542 as well as CDR circuit 544 torespectively provide information on the amount of eye dispersiondistortion and to provide correction information for adjusting fortiming errors due to imperfect clock recovery. The parallel dataproceeds on at 550 to a cross-point switch for rerouting, etc.

FIG. 71A pictorially shows a typical eye diagram 560 for recovered datawhere dotted line 562 shows the center of the eye 560. The upper portion560A of eye carries more noise then the lower portion 560B of eye 560.As a result, the threshold decision level is reduced to be below thecenter 562 of eye 560 to a level, for example, at 563. In this manner,the threshold decision point or offset voltage 566 is set within anarrower window margin 564 for decision threshold since line 563 isshorter than center line 562 of eye 560. However, as provided by thisinvention relative to EDE circuit 542, a wider window margin 574 isachieved, as shown in FIG. 71B through the lowering of the eye center572 relative to the wrap-around on bit boundaries to lower the point ofthe vertical opening of the eye to provide for a lower threshold for abetter margin for lower bit errors due to noise relative to the highernoise on the upper rail 570A of the eye. Also the cross points for thelower rail 570B wrap-around in bit boundaries relative to the horizontalopening of the eye are made lower so that the margin for timing errorsdue, for example, to imperfect clock recovery is wider as indicated at574 in FIG. 71B, for timing determination to read a bit within itsrespective bit boundary, such as indicated at 572. The wider timingmargin in eye 570 as well as a lower threshold below the bit boundarycenter crossing away from the upper eye rail of greatest noise providesfor optimum data recovery. This optimum eye data recovery is provided bythe EDE circuit 542 providing for maximum phase margin at the correctthreshold decision voltage.

Reference is now made to FIG. 72 which is a flowchart illustrating anexample of a process of feedback correction relative to FIG. 70, in thecase here correcting for modulator operating parameters, e.g., biasvoltage and voltage swing to adjust for modulator chirp and extinsionratio, and DFB laser source parameters, e.g., DFB laser intensity orchannel wavelength, through feedback control signal service channel,λ_(S), from the optical receiver to the optical transmitter to reducethe bit error rate (BER). The chirp parameter of a quantum wellelectro-absorption modulator, such as may be modulators 510 in FIG. 70,is a function of the change in absorption characteristics and refractiveindex of the modulator with bias voltage. Typically, a voltage bias maybe selected over a range within which the chirp parameter of themodulator shifts from positive to negative. In a high data rate channelclose to the dispersion limit, a positive chirp increases the BER whilea negative chirp decreases the BER. Similarly, a high extinction ratiotends to decrease the BER while a low extinction ratio tends to increasethe BER. Forward error correction (FEC) decoders 526 in the opticalreceiver are employed to determine the BER of each channel. While thisinformation may be forwarded to the optical transmitter in a variety ofways, it is shown here being transmitted through an optical servicechannel. The modulator operating parameters, e.g., bias voltage andvoltage swing of the modulator, relative to a particular signal channelare adjusted using data relative to the channel BER determined at FECdecoder 526. As illustrated in FIG. 72, BER data is transmitted from theoptical receiver to an optical TxPIC or transceiver via the opticalservice channel shown in FIG. 70. Electronic controller 516 of TxPIC 502employs this data to tune the bias voltage and/or voltage swing of acorresponding modulator 510 to adjust the bias and voltage swing ofmodulation to optimize BER, including optimized BER for the particularfiber type of optical fiber link 506.

With reference to FIG. 72, the BER of the respective received channelsignals is monitored at 580, one channel at a time, but donesequentially at high speed via transmitter and receiver controllers 516and 528 in FIG. 70. As indicated at 582, the FEC encoded information isdecoded at decoders 526 and provided to receiver controller 528, vialines 527 where it is sorted and respectively sent via the servicechannel as signal Xs, on line 531 and received at transmitter controller516 for distribution to modulators 510. Also, in FIG. 70, information isalso sent via line 529 from receiver controller 528 to RxPIC chip 504 totemperature tune its AWG DEMUX 522 via its TEC 521, as well as, forexample, TEC 30A in FIG. 52 or TEC 602 in FIG. 73, next to be discussed,based upon the transmission of one signal channel to match the AWGwavelength grid passband to the wavelength grid of DFB laser sourcearray 512(1) . . . 512(N) of FIG. 70. As shown at 583 in FIG. 72, thechirp or chirp factor, α, of a respective channel modulator 510 isadjusted and, again, the BER for each respective channel is checked(584) and determined whether or not the BER has been reduced to asatisfactory level. A satisfactory level is, for example, a BER below10⁻¹². If yes, other channels at the transmitter TxPIC 502 are checked(585) until all signal channels have satisfactory chirp or α. If no,then adjustment of the signal channel wavelength is accomplished (586)and, again, the BER monitored at the receiver is checked to determine ifit is reduced to an acceptable level. If not, the process is redone,starting with adjustment of the modulator channel chirp (583) followedby channel wavelength channel adjustment (586), if necessary, until anacceptable channel BER level is achieved. When all of the channels havebeen checked (588 to 585) and adjusted relative to both modulator chirpand laser intensity and/or channel wavelength with satisfactory BER (at587 in FIG. 72), the process is complete and the monitoring process forthese laser source and modulator parameters may begin all over again at580.

To be noted that in connection with FIG. 70, the wavelength adjustmentof respective DFB laser sources 508 is made relative to bias changes tothe respective source via drivers 512. However, it is within the scopeof FEC enhanced system 500 to also change the channel wavelength via aDFB laser source heater such as with heaters 208 shown in FIG. 37 ofprovisional application Ser. No. 60/328,207, incorporated herein byreference and as taught in its corresponding non-provisionalapplication, Serial No. 267,331, filed Oct. 8, 2002, such as seen inFIGS. 12-16, filed concurrently herewith and incorporated by reference.

Reference is now made to FIG. 73 depicting RxPIC chip 600 and itsassociated TEC 602. Chip 600 may include at its input 604 from the fiberchannel link an optical amplifier 606 for adding gain to the multiplexedchannel signals after which they provided at first order Brillouin zoneinput to input slab 610 of the arrayed waveguide grating (AWG) 608 viaon-chip optical waveguide 706. AWG 708 includes a plurality of waveguidegratings coupled between input slab 610 and output slab 614 wherein themultiplexed channel signals, as known in the art, are demultiplexed andprovided as an output at a first order Brillouin zone of output slab 614and the respective demultiplexed channel signals are provided on outputwaveguides 616 to respective on-chip PIN photodiodes 622(1) . . .622(12).

As shown in FIG. 73, monitoring PIN photodiodes 624 and 626 arefabricated in the higher order +/− Brillouin zones (e.g., the −1 and +1Brillouin zones) of AWG 608 and are optically coupled to these zones viarespective waveguides 618 and 620. The two photodiodes 624 and 626 areplaced there for the purposes of detection on opposite sides of the AWGpassband. A DFB laser in a TxPIC, such as TxPIC 502 in FIG. 70, isaligned to the passband of AWG 608 when the DFB laser source wavelength,such as wavelength 628 shown in FIGS. 73A and 73B, is tuned such thatphotodiodes 624 and 626 have a balanced output, i.e., their outputs areof the same magnitude on adjacent sides of laser source wavelength 628such as shown at 630 and 632 in FIGS. 73A and 73B.

Referring again to FIG. 73, AWG 608 of RxPIC chip 600 is temperaturetuned via TEC 602 to adjust its passband response to reduce insertionlosses. Photodiodes 624 and 626 may be fabricated to be integrated atthe +/− higher order or first order Brillouin zone output positions ofslab 614. As previously indicated, when the refractive index of AWG 608is properly tuned relative to a selected channel wavelength, there is anindication of equal power in both +/− Brillouin zone photodiodes asillustrated in FIGS. 73A and 734B.

The passband response of AWG 608 will depend upon its refractive indexand, therefore, may be adjusted by temperature tuning. The passbandresponse of AWG 608 may be characterized in the factory to set anoperating temperature of the AWG for which the passband response of theAWG is aligned to a standardized wavelength grid, such as the ITUwavelength channel grid, i.e., the peaks of transmissivity of the AWGare approximately aligned with the desired wavelength channels toachieve acceptable insertion loss levels in the AWG.

While the invention has been described in conjunction with severalspecific embodiments, it is evident to those skilled in the art thatmany further alternatives, modifications and variations will be apparentin light of the foregoing description. For example, beside thedeployment of InGaAsP/InP regime, described relative to the chipstructures for the RxPIC disclosed in this application, the structuresof this invention, the InGaAs/InP regime can also be deployed in thisinvention for the structures for the RxPIC. Also, the number of signalchannels on a TxPIC or RxPIC chip is a matter of choice and skill inachieving tighter spacing of optical components integrated in a PIC chipwhile maintaining tolerable cross-talk levels. Thus, the inventiondescribed herein is intended to embrace all such alternatives,modifications, applications and variations as may fall within the spiritand scope of the appended claims.

1. An optical transmitter photonic integrated circuit (TxPIC)comprising: a semiconductor monolithic chip; a plurality of opticalsignal channels on said chip, each channel comprising a modulated signalsource with its output coupled to an input of an integrated opticalcombiner to form a WDM output signal for transmission off the TxPIC chipat a chip output coupled to an optical transmission link; and an opticalservice channel (OSC) integrated on said TxPIC chip to receive a servicesignal from the optical receiver source also coupled the opticaltransmission link.
 2. The optical transmitter photonic integratedcircuit (TxPIC) of claim 1 wherein said TxPIC chip optical servicechannel (OSC) is coupled at one end as an input of said optical combinerto operate as an output for said service channel received as an inputvia an output of said TxPIC chip through said optical combiner.
 3. Theoptical transmitter photonic integrated circuit (TxPIC) of claim 2wherein the other end of said TxPIC chip optical service channel (OSC)is coupled to a photodetector for converting said service signal from anoptical signal into an electrical signal.
 4. The optical transmitterphotonic integrated circuit (TxPIC) of claim 3 wherein said electricalservice signal is provided to a controller for correction of aperformance parameter on said TxPIC chip.
 5. The optical transmitterphotonic integrated circuit (TxPIC) of claim 4 wherein said performanceparameter is correction of an operational wavelength or output power ofa modulated signal source.
 6. The optical transmitter photonicintegrated circuit (TxPIC) of claim 5 wherein said modulated signalsource comprises an integrated semiconductor laser, or an integratedsemiconductor laser with an integrated externally operated electro-opticmodulator.
 7. The optical transmitter photonic integrated circuit(TxPIC) of claim 4 wherein said performance parameter is correction ofbias point, voltage swing, chirp or extinction ratio of a modulatedsignal source.
 8. The optical transmitter photonic integrated circuit(TxPIC) of claim 7 wherein said modulated signal source comprises anintegrated semiconductor laser, or an integrated semiconductor laserwith an integrated externally operated electro-optic modulator.